Pretend to be riding horse around (3,2)
I believe the answer is:
act up
'horse around' is the definition.
(thesaurus)
'pretend to be riding' is the wordplay.
'pretend' becomes 'act' (I've seen this in another clue).
'to be riding' becomes 'up' (I've seen this in other clues).
'act'+'up'='ACT UP'
(Other definitions for act up that I've seen before include "Misbehave - carry on" , "Behave like a brat" , "Behave badly, cause trouble - like thespian?" , "Mess about" , "create fireworks display?" .)