Mass being performed by essential primate (6)
I believe the answer is:
monkey
'primate' is the definition.
(monkey is a kind of primate)
'mass being performed by essential' is the wordplay.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'being performed' becomes 'on' (I've seen this in other clues).
'by' is a charade indicator (letters next to each other).
'essential' becomes 'key' (I've seen this before).
'm'+'on'+'key'='MONKEY'
(Other definitions for monkey that I've seen before include "Primate with a long tail" , "Naughty child; primate" , "Primate; five hundred (slang)" , "Mischievous type" , "See 10" .)