Breaks the back of bridges (5)
I believe the answer is:
snaps
'breaks' is the definition.
(I know this)
'back of bridges' is the wordplay.
'back of' shows that the letters should be reversed in order (I've seen 'back' mean this).
'bridges' becomes 'spans' ('span' can be a synonym of 'bridge').
'spans' written backwards gives 'SNAPS'.
'the' is the link.
(Other definitions for snaps that I've seen before include "Informal photographs" , "Spells of cold weather" , "shots" , "Breaks under tension" , "Breaks suddenly" .)