A bridge destroyed by army unit (7)
I believe the answer is:
brigade
'army unit' is the definition.
(I know that brigade is a type of army unit)
'a bridge destroyed' is the wordplay.
'destroyed' indicates anagramming the letters.
'a'+'bridge'='abridge'
'abridge' anagrammed gives 'BRIGADE'.
'by' is the link.
(Other definitions for brigade that I've seen before include "Army subdivision" , "Company" , "Corps" , "Part of an army division" , "Abridge (anag)" .)