Nine months? Indeed! (3)
I believe the answer is:
yea
'indeed' is the definition.
(I've seen this in another clue)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for yea that I've seen before include "Affirmative, colloquially" , "Affirmative vote" , "Aye; aye (anag.)" , "Approval" , "Verily" .)