Bird flying around city to upset youngster (4,3)
I believe the answer is:
tiny tot
'youngster' is the definition.
(I know this)
'bird flying around city to upset' is the wordplay.
'bird' becomes 'tit' (tit is a kind of bird).
'flying around' indicates putting letters inside.
'city' becomes 'NY' (short for New York).
'upset' is a reversal indicator.
'to' in reverse letter order is 'ot'.
'tit' going around 'ny' is 'tinyt'.
'tinyt'+'ot'='TINY TOT'
(Other definitions for tiny tot that I've seen before include "Small child" , "baby" , "Very young child" .)