Splitting trousers from behind, weapon ultimately exposing blubber (6)
I believe the answer is:
snivel
'blubber' is the definition.
(synonyms)
'splitting trousers from behind weapon ultimately' is the wordplay.
'splitting' means one lot of letters goes inside another (some letters split a word into two).
'trousers' becomes 'levis' (Levi's is an example).
'from behind' says the letters should be written in reverse.
'ultimately' suggests the final letters.
The last letter of 'weapon' is 'n'.
'levis' written backwards gives 'sivel'.
'sivel' enclosing 'n' is 'SNIVEL'.
'exposing' is the link.
(Other definitions for snivel that I've seen before include "Complain self-pityingly" , "Sniff - whine" , "Blubber contemptibly" , "moan" , "Cry in miserable whining way" .)