Captivate with eleventh rallentando (to some extent) (7)
I believe the answer is:
enthral
'captivate' is the definition.
(I've seen this before)
'with eleventh rallentando to some extent' is the wordplay.
'to some extent' indicates the answer is hidden within the clue.
'ENTHRAL' is hidden in the letters of 'with eleventh rallentando'.
(Other definitions for enthral that I've seen before include "give a spell-binding performance" , "hypnotise" , "Totally fascinate" , "Hold spellbound" , "Bewitch" .)