It fertilises egg at speed (7)
I believe the answer is:
nitrate
'it fertilises' is the definition.
The definition suggests a singular noun which matches the answer.
'egg at speed' is the wordplay.
'egg' becomes 'nit' (a nit is an insect egg).
'at' means one lot of letters go next to another.
'speed' becomes 'rate' (the speed at which something happens).
'nit'+'rate'='NITRATE'
Can you help me to learn more?
(Other definitions for nitrate that I've seen before include "Natural or synthetic fertiliser" , "Tear tin up for fertiliser" , "salt" , "following 28 [SILVER]" , "Nattier (anag.)" .)