Fuss about rapier regularly used for naval weapon (7)
I believe the answer is:
torpedo
'for naval weapon' is the definition.
(I've seen this before)
'fuss about rapier regularly' is the wordplay.
'fuss' becomes 'todo' (both can mean a commotion).
'about' is an insertion indicator.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'rapier' are 'rpe'.
'todo' enclosing 'rpe' is 'TORPEDO'.
'used' acts as a link.
(Other definitions for torpedo that I've seen before include "Weapon travelling under water" , "Poor Ted (anag)" , "Such as could sink" , "Sub's missile" , "Explosive missile from submarine" .)