Otherwise gripped by Handel's 'Esther' (4)
I believe the answer is:
else
'otherwise' is the definition.
('else' can be a synonym of 'otherwise')
'gripped by handel's esther' is the wordplay.
'gripped by' indicates a hidden word.
'ELSE' is hidden within 'handels esther'.
(Other definitions for else that I've seen before include "Alternatively" , "Other than (what is under consideration)" , "Adverb meaning if not, otherwise" , "Eels (anag)" , "As an alternative" .)