It's the last for four months (9)

'for four months' is the definition.
I can't judge whether this defines the answer.

'it's the last' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'the' is 'eth' which is within the answer.
'last' could be 't' (I've seen this in other clues) and 't' is present in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
This may be the basis of the clue (or it may be nonsense).