Pop first of balloons, upsetting relative's son terribly (5)
I believe the answer is:
burst
'pop' is the definition.
(I know that pop can be written as burst)
'first of balloons upsetting relative's son terribly' is the wordplay.
'first of' says to take the initial letters.
The first letters of 'balloons upsetting relatives son terribly' is 'BURST'.
(Other definitions for burst that I've seen before include "Exploded" , "Explode; volley (of fire)" , "Explode, erupt" , "Rupture" , "Explosion" .)