Shortly notice a piece of ammunition that is found on the beach (8)
I believe the answer is:
seashell
'that is found on the beach' is the definition.
(seashells are often found on beaches)
'shortly notice a piece of ammunition' is the wordplay.
'shortly' means to remove the last letter.
'notice' becomes 'see' (notice can mean to spot or see).
'piece of ammunition' becomes 'shell'.
'see' with its last letter removed is 'se'.
'se'+'a'+'shell'='SEASHELL'
(Other definitions for seashell that I've seen before include "scallop?" , "one picked up on shore" , "Hard remains of marine mollusc" , "home by the coast, perhaps" , "periwinkle?" .)