Apply cross-tree contrivance (5)
I believe the answer is:
exert
'apply' is the definition.
(exerting is a kind of applying)
'cross-tree contrivance' is the wordplay.
'cross' becomes 'x' (X is cross-shaped).
'contrivance' indicates anagramming the letters.
'x'+'tree'='xtree'
'xtree' is an anagram of 'EXERT'.
(Other definitions for exert that I've seen before include "Bring into use, strength or influence" , "Strive" , "Bring into play, wield" , "Exercise, wield" , "Bring into operation" .)