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' means one lot of letters goes inside another (some letters go outside others).
'ever' going within '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" .)