Fat: felt anxious, terribly uncertain as to what should be done (2,1,5,2,4)
I believe the answer is:
in a state of flux
'uncertain as to what should be done' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'fat felt anxious terribly' is the wordplay.
'terribly' indicates anagramming the letters.
'fat'+'felt'+'anxious'='fatfeltanxious'
'fatfeltanxious' anagrammed gives 'IN A STATE OF FLUX'.
Can you help me to learn more?
(Other definitions for in a state of flux that I've seen before include "Continuously changing" , "continually changing?" .)