Get together for a series of calls on horseback (5,2)
I believe the answer is:
round up
'get together' is the definition.
(thesaurus)
'series of calls on horseback' is the wordplay.
'series of calls' becomes 'round' (I've seen this before).
'on horseback' becomes 'up' (up can mean up on a horse).
'round'+'up'='ROUND-UP'
'for a' is the link.
(Other definitions for round up that I've seen before include "Use less accurate figure" , "collection of data" , "For convenience, increase the number" , "marginally ahead in boxing bout?" , "herd" .)