Under a colleague, unctuous reprobate initially lay (7)
I believe the answer is:
amateur
'lay' is the definition.
(as in a layman)
'under a colleague unctuous reprobate initially' is the wordplay.
'under' means one lot of letters go next to another (in a down clue, some letters go below others).
'colleague' becomes 'mate' (I've seen this in another clue).
'initially' suggests taking the first letters.
The first letters of 'unctuous reprobate' is 'ur'.
'a' put next to 'mate' is 'amate'.
'amate'+'ur'='AMATEUR'
(Other definitions for amateur that I've seen before include "One is not professional" , "Unprofessional" , "enthusiast" , "Tum area (anag.)" , "Unpaid (sports)" .)