A young hare is always allowed outside (7)
I believe the answer is:
'a young hare' is the definition.
(I've seen this in another clue)
'always allowed outside' is the wordplay.
'always' becomes 'ever' (synonyms).
'allowed' becomes 'let' (to let is to allow or permit).
'outside' means one lot of letters goes inside another.
'ever' going into 'let' is 'LEVERET'.
'is' is the link.
(Other definitions for leveret that I've seen before include "Speedy young thing" , "hare about in spring" , "Baby hare" , "Young hare" , "Little beast" .)