Be keen then disgruntled by employment (7)
I believe the answer is:
enthuse
'be keen' is the definition.
Although both the answer and definition are verbs in their base form, I don't understand how one could define the other.
'then disgruntled by employment' is the wordplay.
I cannot really see how this works, but
an anagram of 'then' is 'enth' which is within the answer.
'employment' could be 'use' (how something is used) and 'use' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for enthuse that I've seen before include "Express eager enjoyment, interest or approval" , "Appear keenly interested" , "Show feeling of eager liking and approval" , "Seen hut? (anag) - show a lively interest" , "Get passionate" .)