The car gears are ruined by an intruder (11)
I believe the answer is:
gatecrasher
'an intruder' is the definition.
(gatecrasher is a kind of intruder)
'the car gears are ruined' is the wordplay.
'are ruined' indicates an anagram.
'the'+'car'+'gears'='thecargears'
'thecargears' anagrammed gives 'GATECRASHER'.
'by' is the link.
(Other definitions for gatecrasher that I've seen before include "unwelcome visitor" , "Uninvited guest" , "Unwelcome guest" .)