Long twelvemonth at end of campaign (5)
I believe the answer is:
yearn
'long' is the definition.
('yearn' can be a synonym of 'long')
'twelvemonth at end of campaign' is the wordplay.
'twelvemonth' becomes 'year' ('year' can be a synonym of 'twelvemonth').
'at end of' suggests the final letters.
The final letter of 'campaign' is 'n'.
'year'+'n'='YEARN'
(Other definitions for yearn that I've seen before include "Feel desire" , "Hanker" , "Desire, want" , "Long - pine" , "Pant - pine" .)