Secluded retreats accommodating small numbers without approval (5)
I believe the answer is:
nooks
'secluded retreats' is the definition.
(I've seen this before)
'numbers without approval' is the wordplay.
'numbers' becomes 'nos' ('no' is an abbreviation for number).
'without' is an insertion indicator ('without' can be similar in meaning to 'outside').
'approval' becomes 'ok' (OK is an example).
'nos' going around 'ok' is 'NOOKS'.
'accommodating small' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for nooks that I've seen before include "secluded parts" , "In security areas" , "Recesses" , "Niches and crannies" , "Alcoves" .)