Best time to get behind (5)
I believe the answer is:
trump
'best' is the definition.
('trump' can be a synonym of 'best')
'time to get behind' is the wordplay.
'time' becomes 't'.
'to get' means one lot of letters go next to another.
'behind' becomes 'rump' ('rump' can be a synonym of 'behind'**).
't'+'rump'='TRUMP'
(Other definitions for trump that I've seen before include "Powerful card - the last one to be heard at the Day of Judgement" , "reliable person" , "Reliable type" , "Outplay" , "Powerful playing card" .)