Month mistaken as year, initially (3)
I believe the answer is:
may
'month' is the definition.
(I've seen this before)
'mistaken as year initially' is the wordplay.
'initially' says to take the initial letters.
The first letters of 'mistaken as year' is 'MAY'.
(Other definitions for may that I've seen before include "plant that's tropical making comeback" , "The Darling Buds of - (H. E. Bates)" , "Might" , "Fifth month" , "Spring" .)