Reinforce part of post if fencing (7)
I believe the answer is:
stiffen
'reinforce' is the definition.
(thesaurus)
'part of post if fencing' is the wordplay.
'part of' says the answer is hidden in the clue.
'STIFFEN' is hidden in the letters of 'post if fencing'.
(Other definitions for stiffen that I've seen before include "Become more rigid" , "more severe" , "Grow harder" , "Add starch" , "Give strength to" .)