Finish second, then first (4)
I believe the answer is:
stop
'finish' is the definition.
('stop' can be a synonym of 'finish')
'second then first' is the wordplay.
'second' becomes 's' (common abbreviation).
'then' is a charade indicator (letters next to each other).
'first' becomes 'top' ('top' can be a synonym of 'first'**).
's'+'top'='STOP'
(Other definitions for stop that I've seen before include "Don't allow" , "Post (anag.)" , "high card in bridge" , "set of organ pipes" , "Red light signal" .)