Cover one in crib (6)
I believe the answer is:
canopy
'cover' is the definition.
(I know that canopy is a type of cover)
'one in crib' is the wordplay.
'one' becomes 'an' (an apple is one apple).
'in' is an insertion indicator.
'crib' becomes 'copy' (I've seen this before).
'an' inserted within 'copy' is 'CANOPY'.
(Other definitions for canopy that I've seen before include "Upper layer of a forest" , "Roof-like awning" , "Highest level of foliage in a forest" , "Hanging cover over bed, shopfront, etc" , "Awning - roofed structure" .)