Ruin a metal bird (6)
I believe the answer is:
martin
'bird' is the definition.
(martin is a kind of bird)
'ruin a metal' is the wordplay.
'ruin' becomes 'mar' (I've seen this in another clue).
'a metal' becomes 'tin' (tin is a kind of metal).
'mar'+'tin'='MARTIN'
(Other definitions for martin that I've seen before include "Creature that flies" , "Short-tailed swallow" , "Bird or boy's name" , "Male name, bird" , "Dickens novel featuring Mr Pecksniff" .)