Second, better check (4)
I believe the answer is:
stop
'check' is the definition.
('stop' can be a synonym of 'check')
'second better' is the wordplay.
'second' becomes 's' (common abbreviation).
'better' becomes 'top' (I've seen this in other clues).
's'+'top'='STOP'
(Other definitions for stop that I've seen before include "End - period" , "Post (anag.)" , "Withhold large amount raised" , "Don't allow" , "This watch is used to time races" .)