Retreats in numbers, receiving permission (5)
I believe the answer is:
nooks
'retreats' is the definition.
(nook is a kind of retreat)
'numbers receiving permission' is the wordplay.
'numbers' becomes 'nos' (no. is a kind of number**).
'receiving' is an insertion indicator.
'permission' becomes 'ok' (what is said to give permission).
'nos' going around 'ok' is 'NOOKS'.
'in' is the link.
(Other definitions for nooks that I've seen before include "secluded parts" , "Alcoves" , "Secluded retreats" , "Recesses" , "Niches and crannies" .)