Gathering US general has left (7)
I believe the answer is:
cluster
'gathering' is the definition.
(thesaurus)
'us general has left' is the wordplay.
'us general' becomes 'Custer' (George Custer, 19th-century US cavalry commander).
'has' is an insertion indicator.
'left' becomes 'l' (common abbreviation).
'custer' placed around 'l' is 'CLUSTER'.
(Other definitions for cluster that I've seen before include "Close group of things" , "Tight group of like things" , "Close grouping" , "group together" , "Come together" .)