Odds on bad weather? That's a wrench (6)
I believe the answer is:
sprain
'wrench' is the definition.
(sprain is a kind of wrench)
'odds on bad weather?' is the wordplay.
'odds' becomes 'sp' (betting abbreviation for starting price).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'bad weather?' becomes 'rain' (rain is an example of bad weather).
'sp'+'rain'='SPRAIN'
'that's a' is the link.
(Other definitions for sprain that I've seen before include "Wrench or twist" , "Pulled muscle" , "Twist ligament(s)" , "Injury to ligaments" , "Rick" .)