Progressed quickly in skill, being thorough (6)
I believe the answer is:
arrant
'thorough' is the definition.
(I've seen this in another clue)
'progressed quickly in skill' is the wordplay.
'progressed quickly' becomes 'ran' (I can't explain this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'skill' becomes 'art' (both can mean a talent).
'ran' put inside 'art' is 'ARRANT'.
'being' is the link.
Can you help me to learn more?
(Other definitions for arrant that I've seen before include "Extreme or flagrant, perhaps nonsense" , "Utter; downright" , "Out-and-out, notorious" , "Downright, out-and-out - say nonsense" , "Out-and-out, utter" .)