Felt bereft as you are possessed by mad demon (7)
I believe the answer is:
mourned
'felt bereft' is the definition.
I can't judge whether this defines the answer.
'you are possessed by mad demon' is the wordplay.
'you are' becomes 'ur' (textspeak abbreviation).
'possessed by' indicates putting letters inside.
'mad' indicates anagramming the letters.
'demon' anagrammed gives 'moned'.
'ur' inserted inside 'moned' is 'MOURNED'.
'as' acts as a link.
Can you help me to learn more?
(Other definitions for mourned that I've seen before include "No demur about the late departed" , "Grieved for, lamented" , "Grieved, sorrowed" .)