A graduate uses his true letters? Not at all professional! (10)
I believe the answer is:
amateurish
'all professional' is the definition.
Both the definition and answer are adjectives. Maybe there's an association between them I don't understand?
'a graduate uses his true letters? not' is the wordplay.
I cannot really understand how this works, but
'a' is within the answer.
'graduate' could be 'ma' (abbreviation for Master of Arts) and 'ma' is present in the answer.
an anagram of 'his' is 'ish' which is present in the answer.
an anagram of 'true' is 'teur' which is within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'at' acts as a link.
Can you help me to learn more?
(Other definitions for amateurish that I've seen before include "Rather 31 across" , "rather inept" , "done imperfectly" , "Lay" , "using little craft, perhaps" .)