Gets very angry about resistance groups (7)
I believe the answer is:
streams
'groups' is the definition.
(stream is a kind of group)
'gets very angry about resistance' is the wordplay.
'gets very angry' becomes 'steams' (someone steaming is very angry).
'about' is an insertion indicator.
'resistance' becomes 'r' (electronics abbreviation).
'steams' enclosing 'r' is 'STREAMS'.
(Other definitions for streams that I've seen before include "Masters (anag.)" , "Torrents" , "Brooks" , "Rills" .)