Long twelve months ending in confusion (5)
I believe the answer is:
yearn
'long' is the definition.
(I know that long can be written as yearn)
'twelve months ending in confusion' is the wordplay.
'twelve months' becomes 'year' (I've seen this in another clue).
'ending in' indicates one should take the final letters.
The final letter of 'confusion' is 'n'.
'year'+'n'='YEARN'
(Other definitions for yearn that I've seen before include "Long - pine" , "Mark of injury" , "Curdle" , "Long (for something)" , "lust" .)