Enid is upset internally (6)
I believe the answer is:
inside
'internally' is the definition.
Both the definition and answer are adverbs. Perhaps you can see an association between them that I can't see?
'enid is upset' is the wordplay.
'upset' indicates anagramming the letters.
'enid'+'is'='enidis'
'enidis' is an anagram of 'INSIDE'.
Can you help me to learn more?
(Other definitions for inside that I've seen before include "Behind bars (sl.)" , "Within - interior" , "(Of information) from a confidential source" , "Middle" , "Inner surface" .)