Father's turn to weaken (3)
I believe the answer is:
sap
'weaken' is the definition.
(I've seen this in another clue)
'father's turn' is the wordplay.
'father's' becomes 'pa's' (pa can mean dad or father).
'turn' says the letters should be written in reverse.
'pas' reversed gives 'SAP'.
'to' acts as a link.
(Other definitions for sap that I've seen before include "Internal tree moisture" , "tunnel" , "The juice in trees" , "Tree's blood" , "Deplete, drain of resources" .)