Rum drinks outside that won't show much return (5,4)
I believe the answer is:
short odds
'won't show much return' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.
'rum drinks outside' is the wordplay.
'rum' becomes 'odd' (rum can mean strange or dubious).
'drinks' becomes 'shorts' (short can mean a drink of spirits).
'outside' is an insertion indicator (some letters go outside others).
'odd' inserted inside 'shorts' is 'SHORT ODDS'.
'that' acts as a link.
Can you help me to learn more?
(Other definitions for short odds that I've seen before include "What favourites are at the bookmaker" , "An almost even chance" .)