Military unit destroyed a bridge (7)
I believe the answer is:
brigade
'military unit' is the definition.
(I know that brigade is a type of army unit)
'destroyed a bridge' is the wordplay.
'destroyed' indicates an anagram.
'a'+'bridge'='abridge'
'abridge' anagrammed gives 'BRIGADE'.
(Other definitions for brigade that I've seen before include "Company" , "Body of troops" , "Army grouping" , "Soldiers" , "Rig a bed (anag)" .)