Left bitter, under pressure but calm (6)
I believe the answer is:
placid
'calm' is the definition.
('placid' can be similar in meaning to 'calm')
'left bitter under pressure' is the wordplay.
'left' becomes 'l' (common abbreviation).
'bitter' becomes 'acid' ('acid' can be a synonym of 'bitter'**).
'under' is a charade indicator (letters next to each other).
'pressure' becomes 'p' (abbreviation).
'l'+'acid'='lacid'
'lacid' put after 'p' is 'PLACID'.
'but' is the link.
(Other definitions for placid that I've seen before include "Unconcerned" , "Calm, peaceful" , "not easily angered" , "Calm and free from disturbance" , "Quiet and still" .)