Watch merit being eroded (5)
I believe the answer is:
timer
'watch' is the definition.
(a watch might be used to time something)
'merit being eroded' is the wordplay.
'being eroded' indicates anagramming the letters.
'merit' with letters rearranged gives 'TIMER'.
(Other definitions for timer that I've seen before include "Maybe watch" , "One may be alarmed" , "Stopwatch, say" , "Clocklike actuator" , "Pre-set switch" .)