University College may be accepted by reformed idle mathematician (6)
I believe the answer is:
euclid
'mathematician' is the definition.
(Euclid is an example)
'university college may be accepted by reformed idle' is the wordplay.
'university' becomes 'u'.
'college' becomes 'c'.
'may be accepted by' is an insertion indicator.
'reformed' is an anagram indicator.
'u'+'c'='uc'
'idle' is an anagram of 'elid'.
'uc' put inside 'elid' is 'EUCLID'.
(Other definitions for euclid that I've seen before include "Ancient Greek mathematician" , "Greek mathematician c. three hundred BC" , "Great Greek mathematician, notable for geometry" , "Ancient Greek master of geometry" , "Old mathematician" .)