Unsettle girl’s jazz group, initially being unusually behind time (14)
I believe the answer is:
discombobulate
'unsettle' is the definition.
(thesaurus)
'girl's jazz group initially being unusually behind time' is the wordplay.
'girl's' becomes 'Dis' (Di is short for Diana).
'jazz group' becomes 'combo' (synonyms).
'initially' indicates taking the first letters.
'behind time' becomes 'late' (I've seen this before).
The initial letters of 'being unusually' is 'bu'.
'dis'+'combo'+'bu'+'late'='DISCOMBOBULATE'
(Other definitions for discombobulate that I've seen before include "Confuse" , "Throw" , "Upset" , "rattle?" , "cause confusion" .)