Found group on horseback (3,2)
I believe the answer is:
set up
'found' is the definition.
(thesaurus)
'group on horseback' is the wordplay.
'group' becomes 'set' (set is a kind of group**).
'on horseback' becomes 'up'.
'set'+'up'='SET-UP'
(Other definitions for set up that I've seen before include "Organise; mount" , "a useful situation at tennis?" , "An arrangement designed to trap or cheat" , "Contrived situation" , "Erect; fix" .)