It stops worker going on main line (7)
I believe the answer is:
sealant
'it stops' is the definition.
The definition suggests a singular noun which matches the answer.
'worker going on main line' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'going on' means one lot of letters go next to another.
'main' becomes 'sea' (the main is an old name for the sea).
'line' becomes 'l' (used when specifying particular lines from a poem).
'sea'+'l'='seal'
'ant' after 'seal' is 'SEALANT'.
Can you help me to learn more?
(Other definitions for sealant that I've seen before include "Material to stop leaks" , "Eg, Fermat or Napier" , "Material stopping leaks" , "Substance filling gap to prevent leakage" , "Substance for closing something tightly" .)