Batting partner who's doing a stretch (6)
I believe the answer is:
inmate
'who's doing a stretch' is the definition.
(someone spending a stretch in prison)
'batting partner' is the wordplay.
'batting' becomes 'in' (as in batting or being 'in' in cricket).
'partner' becomes 'mate' (synonyms).
'in'+'mate'='INMATE'
(Other definitions for inmate that I've seen before include "Con" , "One institutionally confined" , "One held in prison" , "Convict" , "One is in jail" .)