Contract makes cheat switch son for daughter (7)
I believe the answer is:
dwindle
'contract' is the definition.
(thesaurus)
'cheat switch son for daughter' is the wordplay.
I cannot really see how this works, but
'daughter' could be 'd' (genealogical abbreviation) and 'd' is found within the answer.
The remaining letters 'windle' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'makes' acts as a link.
(Other definitions for dwindle that I've seen before include "tail off" , "Grow less - waste away" , "Shrink, grow smaller" , "Die away" , "become weaker" .)