Not keeping time musically? That's unusual (7)
I believe the answer is:
offbeat
'that's unusual' is the definition.
(offbeat means unusual or whacky)
'not keeping time' is the wordplay.
'not keeping' becomes 'off' (I can't justify this - if you can you should believe this answer much more).
'time' becomes 'beat' (beat is a kind of time).
'off'+'beat'='OFFBEAT'
'musically?' is the link.
This may not be correct. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for offbeat that I've seen before include "Unusual, not ordinary - or rhythmic" , "Wacky; syncopated" , "rather unorthodox" , "Unusual or unconventional - perhaps in music" , "Unconventional or lacking rhythm" .)