Bad rhythm that's unusual (7)
I believe the answer is:
offbeat
'unusual' is the definition.
(offbeat means unusual or whacky)
'bad rhythm' is the wordplay.
'bad' becomes 'off' (eg meat that's gone off is bad).
'rhythm' becomes 'beat' (synonyms).
'off'+'beat'='OFFBEAT'
'that's' is the link.
(Other definitions for offbeat that I've seen before include "rather unorthodox" , "Strange" , "Unusual, not ordinary - or rhythmic" , "Unusual or unconventional - perhaps in music" , "Bohemian" .)