Unexpectedly snap bridge (4)
I believe the answer is:
span
'bridge' is the definition.
(I know that bridge can be written as span)
'unexpectedly snap' is the wordplay.
'unexpectedly' is an anagram indicator.
'snap' is an anagram of 'SPAN'.
(Other definitions for span that I've seen before include "Width of pans" , "Length of bridge - 9 inches!" , "Distance or interval between two points" , "Bridge made of pans" , "too short for the foot" .)