One beating master in gruesome murder (7)
I believe the answer is:
drummer
'one beating' is the definition.
(I've seen this before)
'master in gruesome murder' is the wordplay.
'master' becomes 'M' (common abbreviation eg MA, Master of Art).
'in' indicates putting letters inside.
'gruesome' indicates an anagram (I've seen this in other clues).
'murder' is an anagram of 'drumer'.
'm' going into 'drumer' is 'DRUMMER'.
(Other definitions for drummer that I've seen before include "Timpani beater" , "Someone who keeps 18 [RHYTHM]" , "One using sticks" , "player among the percussion" , "one of a band" .)