Apply, as influence (5)
I believe the answer is:
exert
'apply as influence' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for exert that I've seen before include "Bring into operation" , "Bring into use, strength or influence" , "Bring into use, say strength" , "Make a great effort" , "Bring (force) to bear" .)