Top to, initially, bottom (5)
I believe the answer is:
trump
'top' is the definition.
(thesaurus)
'to initially bottom' is the wordplay.
'initially' says to take the initial letters.
'bottom' becomes 'rump' ('rump' can be a synonym of 'bottom').
The initial letter of 'to' is 't'.
't'+'rump'='TRUMP'
(Other definitions for trump that I've seen before include "Card beating other suits" , "Outplay" , "High card" , "Outdo (with a card?)" , "Card of the most powerful suit" .)