Gloss ruined axle pin (7)
I believe the answer is:
explain
'gloss' is the definition.
(thesaurus)
'ruined axle pin' is the wordplay.
'ruined' indicates an anagram.
'axle'+'pin'='axlepin'
'axlepin' is an anagram of 'EXPLAIN'.
(Other definitions for explain that I've seen before include "Make (something) clear" , "Elucidate - define" , "Make understandable" , "Defend" , "Give reason for" .)