Mass on small island for primate (6)
I believe the answer is:
monkey
'for primate' is the definition.
(monkey is a kind of primate)
'mass on small island' is the wordplay.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'small island' becomes 'key' (I've seen this before).
'm'+'on'+'key'='MONKEY'
(Other definitions for monkey that I've seen before include "Primate; five hundred (slang)" , "Small primate" , "Any of various long-tailed primates" , "See 10" , "Chile pine (the Araucaria tree)" .)