Mast at sea trapping Nelson's second arm (5)
I believe the answer is:
spear
'arm' is the definition.
(spear is a kind of arm)
'mast at sea trapping nelson's second' is the wordplay.
'mast at sea' becomes 'spar' (spar can mean a mast on a ship).
'trapping' means one lot of letters goes inside another.
'nelson's second' becomes 'e' (2nd letter of 'nelson').
'spar' placed around 'e' is 'SPEAR'.
(Other definitions for spear that I've seen before include "mint may be added" , "Javelin or harpoon" , "Sharp weapon" , "Lance" , "asparagus for example" .)