Primate’s Mass being performed by principal (6)
I believe the answer is:
monkey
'primate's' is the definition.
(monkey is a kind of primate)
'mass being performed by principal' 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' says to put letters next to each other.
'principal' becomes 'key'.
'm'+'on'+'key'='MONKEY'
(Other definitions for monkey that I've seen before include "Primate; five hundred (slang)" , "Five hundred pounds (colloq.); imp" , "Any of various long-tailed primates" , "Medium-sized primate" , "Long-tailed primate" .)