Pub on Merseyside initially provides humour (6)
I believe the answer is:
phlegm
'humour' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'pub on merseyside initially' is the wordplay.
'pub' becomes 'ph' (abbreviation for Public House).
'on' becomes 'leg' (cricket term, fielding positions).
'initially' suggests taking the first letters.
The first letter of 'merseyside' is 'm'.
'ph'+'leg'+'m'='PHLEGM'
'provides' is the link.
Can you help me to learn more?
(Other definitions for phlegm that I've seen before include "indifference" , "'spit it out!'" , "Coolness" , "Calm temperament" , "Thick mucus in throat" .)