Remember to gather support for reasonable case (9)
I believe the answer is:
recollect
'remember' is the definition.
('recollect' can be a synonym of 'remember')
'gather support for reasonable case' is the wordplay.
'gather' becomes 'collect' ('collect' can be a synonym of 'gather'**).
'support for' means one lot of letters go next to another.
'case' means to remove the middle letters.
'reasonable' with its centre removed is 're'.
'collect' put after 're' is 'RECOLLECT'.
'to' is the link.
(Other definitions for recollect that I've seen before include "Reminisce" , "Remember" , "Flock together again" , "Recall to mind" , "in Scotland, mind" .)