It's a job, after one short month, to compose a trio (7)
I believe the answer is:
janitor
'it's a job' is the definition.
I can't judge whether this defines the answer.
'after one short month to compose a trio' is the wordplay.
'after' is a charade indicator (letters next to each other).
'one short month' becomes 'Jan' (Jan is short for January; 'short' indicates a short form).
'to compose' indicates anagramming the letters.
'trio' is an anagram of 'itor'.
'jan' put next to 'itor' is 'JANITOR'.
Can you help me to learn more?
(Other definitions for janitor that I've seen before include "Caretaker (mainly US)" , "One employed to clean and maintain a building" , "Caretaker or doorkeeper" , "Person looking after building" , "(In Scotland) school caretaker" .)