Compete to trap unpleasant child in swing (7)
I believe the answer is:
vibrate
'swing' is the definition.
(vibrating is a kind of swinging)
'compete to trap unpleasant child' is the wordplay.
'compete' becomes 'vie' ('vie' can be a synonym of 'compete').
'to trap' is an insertion indicator.
'unpleasant child' becomes 'brat' (I've seen this before).
'vie' enclosing 'brat' is 'VIBRATE'.
'in' is the link.
(Other definitions for vibrate that I've seen before include "Move rapidly to and fro" , "swing" , "tremble" , "Quiver rapidly, resonate" , "Oscillate rapidly" .)