Avert corrosion in submarine brought to loch (9)
I believe the answer is:
underseal
'avert corrosion' is the definition.
'submarine brought to loch' is the wordplay.
'submarine' becomes 'undersea' (submarine means under the sea).
'brought to' says to put letters next to each other.
'loch' becomes 'l'.
'undersea'+'l'='UNDERSEAL'
'in' acts as a link.
(Other definitions for underseal that I've seen before include "protection against the weather" , "Protective coat" , "waterproof coat" , "Waterproofing" , "Protection against corrosion in car" .)