Furious attack to secure bridgehead (5)
I believe the answer is:
rabid
'furious' is the definition.
(I've seen this in another clue)
'attack to secure bridgehead' is the wordplay.
'attack' becomes 'raid' (raiding is a kind of attacking).
'to secure' means one lot of letters goes inside another (inserted letters are secured inside).
'bridgehead' becomes 'b' (head letter of 'bridge').
'raid' placed around 'b' is 'RABID'.
(Other definitions for rabid that I've seen before include "Crazy" , "Furious like mad dog?" , "Ill from a mad dog bite?" , "Extremely fanatical" , "extremist" .)