Groups associated with US general left trapped (8)
I believe the answer is:
clusters
'groups' is the definition.
The answer is an example.
'associated with us general left trapped' is the wordplay.
'associated with us general' becomes 'Custer's' (as in Custer's Last Stand, 1876 battle).
'left' becomes 'L' (common abbreviation).
'trapped' is an insertion indicator.
'custers' enclosing 'l' is 'CLUSTERS'.
(Other definitions for clusters that I've seen before include "Groups or bunches of similar things" , "Bunches, clumps" , "Small groups or gatherings" , "Numbers of flowers growing together on one stem" , "Curls set in small bunches" .)