「敫」 does not have a solid basis for the meaning ancient musical instrument, but it looks like that meaning was derived from a transcription or translation error for a word which really did represent an ancient wind instrument. I'll present two interpretations as to where I think this translation originated from, and actually either interpretation arrives at the same conclusion.
First interpretation: this is a mistranslation of the Shuowen Jiezi definition.
「敫」: flowing sunlight. From semantic 「白」 and semantic 「放」. Read like 「龠」.
The ancient musical instrument definition is given at Unicode, and interprets the last sentence 讀若龠 as imparting both meaning and sound of 「龠」 (original character of 「籥」, as specified at the Wikipedia page as a kind of ancient flute), implying that 「敫」 (Zhengzhang OC: /*kleːwɢs/) is a rebus of 「龠」 (/*lowɢ/).
Actually, no such usage of 「敫」 as 「龠」 (籥) seems to be recorded in literature, so 讀若龠 should be interpreted as having a similar/same sound to 「龠」 rather than is a rebus of 「龠」. I would conclude that Unicode's English definition is wrong.
讀若Ｘ is a bit controversial in its interpretation, so there's probably more misunderstandings in some of Unicode's definitions.
For reference, Shuowen's definition of flowing sunlight is a phonetic loan, and its character analysis is incorrect. This definition probably refers to a word cognate to or the same as 「耀」 (/*lewɢs/) instead.
「敫」 is constructed from semantic 「皃」 (appearance, here representing something/someone else) and semantic 「攴・攵」 (to hit), indicating the meaning to strike/hit something or someone else. The bottom part of 「皃」 (儿・人) was later corrupted into 「方」.
「敫」 is the original character of 「撽」, and is a similar or the same word as 「敲」.
Second interpretation: 「敫」 was misidentified as 「㰾」 during data entry (they kind of look similar), which Unicode also gives a definition of
a kind of music instrument in ancient times
Thankfully, this definition has a more solid basis. From Zhengzitong:
Today (time of authorship of Zhengzitong), there are instruments of the type 「壎」 and 「箎」 (ancient wind instruments) called 「㰾子」...