Fixed month in effect rare (8)
I believe the answer is:
doctored
'fixed' is the definition.
('doctor' can be a synonym of 'fix')
'month in effect rare' is the wordplay.
I cannot really understand how this works, but
'month' could be 'oct' (Oct is an example) and 'oct' is found within the answer.
'effect' could be 'do' (synonyms) and 'do' is found in the answer.
'rare' could be 'r' (listed by Oxford Dictionary of Abbreviations) and 'r' is found within the answer.
The remaining letters 'ed' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
(Other definitions for doctored that I've seen before include "Patched up or falsified - like medical man?" , "Adulterated" , "Contaminated" .)