One down and out to gain nothing (4)
I believe the answer is:
wino
'one down and out' is the definition.
(both can mean a homeless person)
'gain nothing' is the wordplay.
'gain' becomes 'win' (I've seen this before).
'nothing' becomes 'o' (looks like zero - 0).
'win'+'o'='WINO'
'to' is the link.
(Other definitions for wino that I've seen before include "heavy drinker" , "Street drunkard" , "Habitual drunk" , "Lush" , "Down-and-out drunkard" .)