Gloomy note disrupted study of insects (10)
I believe the answer is:
entomology
'study of insects' is the definition.
(entomology is the study of insects)
'gloomy note disrupted' is the wordplay.
'disrupted' indicates an anagram.
'gloomy'+'note'='gloomynote'
'gloomynote' anagrammed gives 'ENTOMOLOGY'.
(Other definitions for entomology that I've seen before include "Insect study" , "in which cricket may be studied" , "Study of insects" , "Science" , "cricket field, for example" .)