Trumpeter's one was back to front and short (4)
I believe the answer is:
swan
'trumpeter's' is the definition.
(I've seen this in another clue)
'one was back to front and short' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'was' becomes 'ws' (I am not sure about this - if you are sure you should believe this answer much more).
'back to front' shows that the letters should be reversed in order.
'and short' becomes 'n' (common abbreviation for 'and'; 'short' indicates a short form).
'a'+'ws'='aws'
'aws' backwards is 'swa'.
'swa'+'n'='SWAN'
Can you help me to learn more?
(Other definitions for swan that I've seen before include "Whooper, perhaps" , "Large waterbird" , "Shakespeare is called The .... of Avon" , "Cob - pen" , "Large, web-footed bird" .)