Eyesore seen in rusty engines (4)
I believe the answer is:
stye
'eyesore' is the definition.
(I've seen this before)
'in rusty engines' is the wordplay.
'in' indicates a hidden word.
'STYE' can be found hidden inside 'rusty engines'.
'seen' is the link.
(Other definitions for stye that I've seen before include "dump" , "irritation to the viewer" , "Eye blemish" , "Viewer's complaint" , "Ocular inflammation" .)