Apply (3)
'apply' is the definition.
(synonyms)
This is all the clue.
(Other definitions for use that I've seen before include "Employ (something)" , "Custom - service" , "Employment; utilise" , "Put to a particular purpose" , "Consume; deploy" .)
'apply' is the definition.
(exerting is a kind of applying)
This is the entire clue.
(Other definitions for exert that I've seen before include "Bring into use, say strength" , "Bring into use, apply" , "Bring to bear, perhaps influence" , "Bring into use, strength or influence" , "Bring (force) to bear" .)
(Other definitions for enter that I've seen before include "Pierce" , "Record (in a log)" , "Fill in" , "Take part in a contest" , "Gain access to" .)
'apply' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for refer that I've seen before include "Hand over for decision" , "Make allusion, direct attention with words" , "Bring up" , "Make an allusion, or send on" , "Freer (anag.)" .)