Young runner always allowed outside (7)
I believe the answer is:
leveret
'young runner' is the definition.
(I've seen this before)
'always allowed outside' is the wordplay.
'always' becomes 'ever' (synonyms).
'allowed' becomes 'let' (to let is to allow or permit).
'outside' indicates putting letters inside (some letters go outside others).
'ever' put into 'let' is 'LEVERET'.
(Other definitions for leveret that I've seen before include "Young hare" , "Young three dn" , "Young runner" , "Young animal" , "First-year hare" .)