Monkeys in time upset penniless people (5)
I believe the answer is:
troop
'monkeys in time' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'upset penniless people' is the wordplay.
'upset' is a reversal indicator.
'penniless' becomes 'poor' (similar in meaning).
'people' becomes 't' (I can't justify this - if you can you should give a lot more credence to this answer).
'poor'+'t'='poort'
'poort' back-to-front is 'TROOP'.
Can you help me to learn more?
(Other definitions for troop that I've seen before include "Band of people" , "(Of people) come or go in large numbers" , "Military group" , "Group of people or animals" , "A company of people, soldiers or scouts" .)