Uriah's mate, terribly unprofessional (10)
I believe the answer is:
amateurish
'unprofessional' is the definition.
(I've seen this before)
'uriah's mate terribly' is the wordplay.
'terribly' is an anagram indicator.
'uriahs'+'mate'='uriahsmate'
'uriahsmate' is an anagram of 'AMATEURISH'.
(Other definitions for amateurish that I've seen before include "done imperfectly" , "incompetent" , "Rather 31 across" , "using little craft, perhaps" , "Unprofessional" .)