Chap trains monkeys (9)
I believe the answer is:
mandrills
'monkeys' is the definition.
(I know this)
'chap trains' is the wordplay.
'chap' becomes 'man' (chap can mean a man).
'trains' becomes 'drills' (drilling is a kind of training).
'man'+'drills'='MANDRILLS'
(Other definitions for mandrills that I've seen before include "Animals" , "primates" , "Monkeys" .)