Always allow out a youngster (7)
I believe the answer is:
'a youngster' is the definition.
(I've seen this before)
'always allow out' is the wordplay.
'always' becomes 'ever' (synonyms).
'allow' becomes 'let' (synonyms).
'out' is an insertion indicator.
'ever' going within 'let' is 'LEVERET'.
(Other definitions for leveret that I've seen before include "Small hopper" , "Hare under a year old" , "Baby hare" , "Young animal" , "Young hare" .)