Reputable type of milk opened by puritan (7)
I believe the answer is:
upright
'reputable' is the definition.
(thesaurus)
'type of milk opened by puritan' is the wordplay.
'type of milk' becomes 'UHT' (ultra-high-temperature processing milk).
'opened by' indicates putting letters inside (one word creates an opening in another word).
'puritan' becomes 'prig' (both can mean a prudish person).
'uht' enclosing 'prig' is 'UPRIGHT'.
(Other definitions for upright that I've seen before include "Type of piano" , "Standing straight up" , "deserving respect" , "Honest; goal-post" , "Erect - honest" .)