Firm can shut when ruined (7)
I believe the answer is:
staunch
'firm' is the definition.
(thesaurus)
'can shut when ruined' is the wordplay.
'when ruined' indicates anagramming the letters.
'can'+'shut'='canshut'
'canshut' with letters rearranged gives 'STAUNCH'.
(Other definitions for staunch that I've seen before include "act to stop bleeding" , "solid" , "Faithful and true" , "Check" , "Stout" .)