Said it's a key factor in pollination (3)
I believe the answer is:
bee
'factor in pollination' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'said it's a key' is the wordplay.
'said' shows a homophone (sound like).
'it's a key' becomes 'b' (eg B major, musical key).
'b' sounds like 'BEE'.
Can you help me to learn more?
(Other definitions for bee that I've seen before include "Winged insect" , "Hairy-bodied insect" , "Flying insect (often busy)" , "Social gathering" , "competition" .)