According to Wiedenhof's A Grammar of Mandarin, page 25,
xiǎng xiǎng ‘keep on thinking’ doesn't show tone sandhi because the tones belong to separate clauses, unlike [xiáng xiǎng] ‘think for a moment’.
What meaning of "clause" is used here?
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I don't think there are any clauses. Both of them are verbs. The different tones express different emotions.
If you say "xiǎng xiǎng", you are emphasizing you are pondering. So you pronounce with stress. For instance, you are quarrelling with someone, and you said "Shut up! Let me think about it!"
And if you say "xiáng xiǎng", most of the time you are imagining (A devil whispering beside you ear "Just think about it, the most beautiful women will be yours"), but some of the time you are pondering (Because you are in the duration of deep thinking, and you don't care your questioner very much, and you are carelessly murmuring).