Birds ruin metal containers (7)
I believe the answer is:
martins
'birds' is the definition.
(martin is a kind of bird)
'ruin metal containers' is the wordplay.
'ruin' becomes 'mar' (I've seen this before).
'metal containers' becomes 'tins'.
'mar'+'tins'='MARTINS'
(Other definitions for martins that I've seen before include "St - - , one of the Scilly Isles" , "Eaves-nesting birds" .)