Sack a blackleg, ultimately splitting party (6)
I believe the answer is:
ravage
'sack' is the definition.
(I've seen this before)
'a blackleg ultimately splitting party' is the wordplay.
'ultimately' indicates one should take the final letters.
'splitting' means one lot of letters goes inside another.
'party' becomes 'rave' (I've seen this before**).
The final letter of 'blackleg' is 'g'.
'a'+'g'='ag'
'ag' put into 'rave' is 'RAVAGE'.
(Other definitions for ravage that I've seen before include "Pillage or lay waste" , "Greatly damage, destroy" , "Wreak havoc upon" , "Devastate, damage extensively" , "Lay waste, plunder" .)