Ask about one monkey (4)
I believe the answer is:
saki
'monkey' is the definition.
(I know that saki is a type of platyrrhine)
'ask about one' is the wordplay.
'about' is an anagram indicator.
'one' becomes 'i' (Roman numeral).
'ask' is an anagram of 'sak'.
'sak'+'i'='SAKI'
(Other definitions for saki that I've seen before include "Japanese beverage" , "Short-story writer" , "Rice < monkey" , "Pseudonym of author HH Munro" , "drink in Tokyo" .)