At heart, Enid is upset (6)
I believe the answer is:
inside
'at heart' is the definition.
('inside' can be a synonym of 'at heart')
'enid is upset' is the wordplay.
'upset' indicates anagramming the letters.
'enid'+'is'='enidis'
'enidis' with letters rearranged gives 'INSIDE'.
(Other definitions for inside that I've seen before include "Internal" , "Within - interior" , "(Of information) from a confidential source" , "serving porridge" , "Not external, perhaps in prison" .)