No top on ruined bridge (7)
I believe the answer is:
pontoon
'bridge' is the definition.
(pontoon is a type of bridge)
'no top on ruined' is the wordplay.
'ruined' is an anagram indicator.
'no'+'top'+'on'='notopon'
'notopon' with letters rearranged gives 'PONTOON'.
(Other definitions for pontoon that I've seen before include "Floating bridge support" , "variety of bridge" , "Vingt-et-un" , "It spans" , "Lighter" .)