Mean time retained in stir (12)
I believe the answer is:
intermediate
'mean' is the definition.
(thesaurus)
'time retained in stir' is the wordplay.
'in stir' indicates an anagram (I've seen this in other clues).
'time'+'retained'='timeretained'
'timeretained' with letters rearranged gives 'INTERMEDIATE'.
(Other definitions for intermediate that I've seen before include "Lying between two extremes" , "Act as agent" , "Coming between (eg, in size)" , "Coming between two things in some way" .)