... if they stimulate one of them (7)
I believe the answer is:
whether
'if' is the definition.
(I've seen this before)
'stimulate one of them' is the wordplay.
'stimulate' becomes 'whet' (whetting is a kind of stimulating).
'one of them' becomes 'her' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'whet'+'her'='WHETHER'
'they' acts as a link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for whether that I've seen before include "- - 'tis nobler in the mind... Shakespeare's 'Hamlet'" , "Conjunction indicating choice" , "Expression of doubt" , "-- or no means in any event" , "If in fact" .)