Think fool has got you and setter (6)
I believe the answer is:
assume
'think' is the definition.
(thesaurus)
'fool has got you and setter' is the wordplay.
'fool has' becomes 'ass' (I can't justify this - if you can you should give a lot more credence to this answer).
'got' means one lot of letters go next to another.
'you' becomes 'u' (I've seen this in other clues).
'and' is a charade indicator (letters next to each other).
'setter' becomes 'me' (the setter of the clue).
'ass'+'u'+'me'='ASSUME'
Can you help me to learn more?
(Other definitions for assume that I've seen before include "Guess" , "Suppose; put on" , "Adopt or take for granted" , "Claim without justification" , "Take as read" .)