Avert corrosion in submarine, large (9)
I believe the answer is:
underseal
'avert corrosion' is the definition.
'submarine large' is the wordplay.
'submarine' becomes 'undersea' ('undersea' can be a synonym of 'submarine').
'large' becomes 'l'.
'undersea'+'l'='UNDERSEAL'
'in' acts as a link.
(Other definitions for underseal that I've seen before include "waterproof coat" , "Protective coat" , "Waterproofing" , "Protection against corrosion in car" , "protection against the weather" .)