Spots of corrosion finally found in service (4)
I believe the answer is:
acne
'spots' is the definition.
(skin condition causing spots)
'corrosion finally found in service' is the wordplay.
'finally' says to take the final letters.
'found in' is an insertion indicator.
'service' becomes 'ace' (ace is a kind of service).
The last letter of 'corrosion' is 'n'.
'n' going within 'ace' is 'ACNE'.
'of' acts as a link.
(Other definitions for acne that I've seen before include "Teenage skin blemish" , "(Inflammation causing) spots" , "Skin rash" , "Pimply skin condition of adolescence" , "Teenage spots" .)