Mad military operation to capture bridgehead (5)
I believe the answer is:
rabid
'mad' is the definition.
(I've seen this before)
'military operation to capture bridgehead' is the wordplay.
'military operation' becomes 'raid' (a military attack).
'to capture' is an insertion indicator.
'bridgehead' becomes 'b' (head letter of 'bridge').
'raid' placed around 'b' is 'RABID'.
(Other definitions for rabid that I've seen before include "Extremely fanatical" , "Extreme, fanatical" , "Fanatical like mad dog" , "Fanatical, infected" , "Wild" .)