Stem offering firm support (7)
I believe the answer is:
staunch
This clue is a double definition.
'stem' is the first definition.
(I know this)
'offering firm support' is the second definition.
(as in a staunch supporter)
(Other definitions for staunch that I've seen before include "Plug" , "Trustworthy" , "Firm in loyalty" , "Sturdy" , "way to limit bloodshed" .)