Split second to enter coin (6)
I believe the answer is:
bisect
'split' is the definition.
(thesaurus)
'second to enter coin' is the wordplay.
'second' becomes 'sec' ('sec' can be a synonym of 'second').
'to enter' is an insertion indicator.
'coin' becomes 'bit' (I've seen this before).
'sec' put into 'bit' is 'BISECT'.
(Other definitions for bisect that I've seen before include "Divide in half" , "fork" , "Divide into two equal parts" , "equally split" , "Cut in two equal parts" .)