Lacking the go-ahead for recess? (4)
I believe the answer is:
nook
'for recess?' is the definition.
(a nook is an architectural recess)
'lacking the go-ahead' is the wordplay.
'lacking' becomes 'no' (eg lacking money means with no money).
'the go-ahead' becomes 'ok' (to OK something is to give it the go-ahead).
'no'+'ok'='NOOK'
(Other definitions for nook that I've seen before include "Cosy corner" , "small secure zone" , "Hidey-hole" , "Corner or small recess" , "Cubbyhole" .)