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It's the last for four months (9)

Ross

I believe the answer is:

thirtieth

I'm a little stuck... Click here to teach me more about this clue!

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

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

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