Brothers promise to carry occasionally handy weapon (10)
I believe the answer is:
broadsword
'weapon' is the definition.
(broadsword is a kind of weapon)
'brothers promise to carry occasionally handy' is the wordplay.
'brothers' becomes 'bros'.
'promise' becomes 'word' (as in 'I give you my word').
'to carry' means one lot of letters goes inside another.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
The alternating letters of 'handy' are 'ad'.
'bros'+'word'='brosword'
'brosword' going around 'ad' is 'BROADSWORD'.
(Other definitions for broadsword that I've seen before include "cutter" , "cutting weapon" , "means of attack" , "old weapon" , "Wide-bladed weapon" .)