Whenever inmate strayed outside the yard (7)
I believe the answer is:
anytime
'whenever' is the definition.
(thesaurus)
'inmate strayed outside the yard' is the wordplay.
'strayed' indicates anagramming the letters.
'outside' is an insertion indicator (some letters go outside others).
'the yard' becomes 'y' (abbreviation).
'inmate' with letters rearranged gives 'antime'.
'antime' going around 'y' is 'ANYTIME'.
(Other definitions for anytime that I've seen before include "A tiny me (anag.)" , "At whatever moment" , "Whenever, as it doesn't matter" , "date not fixed" , "Whenever you like" .)