Submarine at first lacks rust protection (9)
I believe the answer is:
underseal
'rust protection' is the definition.
(for automobiles)
'submarine at first lacks' is the wordplay.
'submarine' becomes 'undersea' (submarine means under the sea).
'at' means one lot of letters go next to another.
'first lacks' becomes 'l' (1st letter of 'lacks').
'undersea'+'l'='UNDERSEAL'
(Other definitions for underseal that I've seen before include "Protective coat" , "waterproof coat" , "Waterproofing" , "Protection against corrosion in car" , "protection against the weather" .)