Bad rhythm is unorthodox (7)
I believe the answer is:
offbeat
'unorthodox' is the definition.
(I've seen this before)
'bad rhythm' is the wordplay.
'bad' becomes 'off' (I've seen this before).
'rhythm' becomes 'beat' (synonyms).
'off'+'beat'='OFFBEAT'
'is' acts as a link.
(Other definitions for offbeat that I've seen before include "Wacky, eccentric" , "Strikingly unconventional" , "Wacky; syncopated" , "Unusual, not ordinary - or rhythmic" , "Quirky" .)