Twist betting on weather (6)
I believe the answer is:
sprain
'twist' is the definition.
(I know that sprain is a type of twist)
'betting on weather' is the wordplay.
'betting' becomes 'sp'.
'on' says to put letters next to each other (in a down clue, letters appear on others).
'weather' becomes 'rain' (rain is a kind of weather).
'sp'+'rain'='SPRAIN'
(Other definitions for sprain that I've seen before include "Turn" , "sign of strain" , "unwelcome twist" , "Strain (eg, ankle)" , "Slight muscle injury, eg to ankle" .)