Bitter about essence of Captain Scarlet possibly being changed (7)
I believe the answer is:
altered
'being changed' is the definition.
('altered' can be similar in meaning to 'changed')
'bitter about essence of captain scarlet' is the wordplay.
'bitter' becomes 'ale' (type of beer or bitter).
'about' indicates putting letters inside.
'essence of' indicates the central letters.
'scarlet' becomes 'red' (synonyms).
The central letter of 'captain' is 't'.
'ale' placed around 't' is 'alte'.
'alte'+'red'='ALTERED'
'possibly' is the link.
(Other definitions for altered that I've seen before include "Change" , "Reformed" , "Converted" , "Changed, modified" , "Changed, adapted" .)