Twelve months is long, endless (4)
I believe the answer is:
year
'twelve months' is the definition.
(I've seen this before)
'long endless' is the wordplay.
'long' becomes 'yearn' (synonyms).
'endless' means to remove the last letter.
'yearn' with its final letter taken away is 'YEAR'.
'is' is the link.
(Other definitions for year that I've seen before include "Period of just over 52 weeks" , "See 9" , "Orbital period" , "See 19" , "Manage" .)