Paints flange in a sprocket at first (7)
I believe the answer is:
primers
'paints' is the definition.
(primer is a kind of paint)
'flange in a sprocket at first' is the wordplay.
'flange' becomes 'rim' ('rim' can be a synonym of 'flange').
'in' indicates putting letters inside.
'a' becomes 'per'.
'at first' suggests taking the first letters.
The first letter of 'sprocket' is 's'.
'rim' placed into 'per' is 'primer'.
'primer'+'s'='PRIMERS'
(Other definitions for primers that I've seen before include "Introductory books or undercoats" , "detonators" .)