Use ash perhaps to cover cross up (5)
I believe the answer is:
exert
'use' is the definition.
(I know that exert is a type of use)
'ash perhaps to cover cross up' is the wordplay.
'ash perhaps' becomes 'tree' (ash is a kind of tree).
'to cover' indicates putting letters inside.
'cross' becomes 'x' (X is cross-shaped).
'up' is a reversal indicator (in down clue: letters go upwards).
'tree' enclosing 'x' is 'trexe'.
'trexe' backwards is 'EXERT'.
(Other definitions for exert that I've seen before include "Bring quality or influence into play" , "Apply (force)" , "Make use of" , "Apply (pressure)" , "Bring into use, apply" .)