Constant check (7)
I believe the answer is:
staunch
I believe this clue is a double definition.
'constant' is the first definition.
('staunch' can be similar in meaning to 'constant')
'check' is the second definition.
(I've seen this before)
(Other definitions for staunch that I've seen before include "act to stop bleeding" , "Very loyal, committed" , "Firm; stem" , "Resolute" , "Sturdy" .)