For once, said no (3)
I believe the answer is:
nee
'for once' is the definition.
I can't judge whether this definition defines the answer.
'said no' is the wordplay.
'said' indicates a 'sounds like' (homophone) clue.
'no' becomes 'nay' (I've seen this before).
'nay' is a homophone of 'NEE'.
Can you help me to learn more?
(Other definitions for nee that I've seen before include "Indicating the birth name of a married woman" , "named as 14 [FIANCEE] ?" , "Born as (former name)" , "Used to indicate the maiden name of a married woman" , "indicate old name" .)