Serious tea-user reformed (7)
I believe the answer is:
austere
'serious' is the definition.
(I've seen this before)
'tea-user reformed' is the wordplay.
'reformed' indicates anagramming the letters.
'tea'+'user'='teauser'
'teauser' is an anagram of 'AUSTERE'.
(Other definitions for austere that I've seen before include "Stark" , "Lacking comforts; strict" , "Stern, grave" , "Severe; lacking luxuries" , "Grave, unsmiling" .)