Lots of people, some upsetting bird (4)
I believe the answer is:
myna
'bird' is the definition.
(myna is a kind of bird)
'lots of people some upsetting' is the wordplay.
'lots of' becomes 'm'.
'people some' becomes 'any' (I can't explain this - if you can you should give a lot more credence to this answer).
'upsetting' shows that the letters should be reversed in order.
'any' in reverse letter order is 'yna'.
'm'+'yna'='MYNA'
Can you help me to learn more?
(Other definitions for myna that I've seen before include "Asiatic bird imitating human speech" , "Speech-mimicking bird" , "Bird good at mimicry" , "Bird that's a good mimic" , "Asian bird" .)