Nut was obliged to protect kernel in general (4)
I believe the answer is:
head
'nut' is the definition.
(thesaurus)
'was obliged to protect kernel in general' is the wordplay.
'was obliged' becomes 'had' (to have to do something is to be obliged to do it).
'to protect' is an insertion indicator.
'kernel in' means to look at the middle letters (a kernel is the middle of something).
The centre of 'general' is 'e'.
'had' going around 'e' is 'HEAD'.
(Other definitions for head that I've seen before include "bean" , "Noggin" , "Chief; part of body" , "River source; promontory" , "''and still the wonder grew / That one small ... could carry all he knew'' (Goldsmith)" .)