Bet TV presenter takes drug (4)
I believe the answer is:
ante
'bet' is the definition.
(ante is a kind of bet)
'tv presenter takes drug' is the wordplay.
'tv presenter' becomes 'ant' (one half of Ant and Dec).
'takes' means one lot of letters go next to another.
'drug' becomes 'e' (e is another name for ecstasy).
'ant'+'e'='ANTE'
(Other definitions for ante that I've seen before include "advance payment" , "'Behind, before (4)'" , "expected from stake" , "Money put at risk" , "Poker pot starter" .)