Gather together in circle, on horseback (5,2)
I believe the answer is:
round up
'gather together' is the definition.
'circle on horseback' is the wordplay.
'circle' becomes 'round' ('round' can be a synonym of 'circle'**).
'on horseback' becomes 'up'.
'round'+'up'='ROUND-UP'
'in' is the link.
(Other definitions for round up that I've seen before include "herd" , "Gather together" , "gathering" , "collection of data" , "marginally ahead in boxing bout?" .)