Agree to wear back-to-front (3)
I believe the answer is:
nod
'agree to' is the definition.
(thesaurus)
'wear back-to-front' is the wordplay.
'wear' becomes 'don' ('don' can be a synonym of 'wear').
'back-to-front' is a reversal indicator.
'don' in reverse letter order is 'NOD'.
(Other definitions for nod that I've seen before include "Gesture; agree" , "Signify assent" , "even Homer sometimes does" , "Show agreement (or sleepiness?)" , "It's said to be as good as a wink" .)