Increasingly bad-tempered doctor swallowing drink that is initially relaxing (8)
I believe the answer is:
grumpier
'increasingly bad-tempered doctor swallowing' is the definition.
I can't judge whether this defines the answer.
'drink that is initially relaxing' is the wordplay.
'drink' becomes 'grump' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'initially' indicates taking the first letters.
The first letter of 'relaxing' is 'r'.
'grump'+'ie'+'r'='GRUMPIER'
Can you help me to learn more?
(Another definition for grumpier that I've seen is " Prime rug (anag.)".)