Play about or upset an MP (4)
I believe the answer is:
romp
'play' is the definition.
(romping is a kind of playing)
'or upset an mp' is the wordplay.
'upset' shows that the letters should be reversed in order.
'or' in reverse letter order is 'ro'.
'ro'+'mp'='ROMP'
'about' is the link.
(Other definitions for romp that I've seen before include "Easy victory; play around energetically" , "Play energetically and roughly" , "Boisterous caper" , "Cut capers" , "sexy fun" .)