Ill at ease, throw a fit when forced to enter (6)
I believe the answer is:
shifty
'ill at ease' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I can't see?
'throw a fit when forced to enter' is the wordplay.
'throw' becomes 'shy' (shy is a kind of throw).
'when forced' indicates an anagram.
'to enter' indicates putting letters inside.
'fit' with letters rearranged gives 'ift'.
'shy' going around 'ift' is 'SHIFTY'.
Can you help me to learn more?
(Other definitions for shifty that I've seen before include "Evasive, not straightforward in manner" , "Suspicious-looking" , "Evasive, sneaky" , "Sly, shady" , "Sneaky and sly" .)