Joker is calmer, say, with Penguin first breaking in (3-6)
I believe the answer is:
leg-puller
'joker' is the definition.
(I have seen 'Practical joker' mean 'leg-puller' so perhaps 'joker' could also mean 'leg-puller')
'calmer say with penguin first breaking in' is the wordplay.
'calmer' becomes 'luller' (I can't explain this - if you can you should give a lot more credence to this answer).
'say' becomes 'eg' (both can mean for example).
'with' says to put letters next to each other.
'penguin first' becomes 'p' (lst letter of 'penguin').
'breaking in' is an insertion indicator.
'eg'+'p'='egp'
'luller' placed around 'egp' is 'LEG-PULLER'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for leg-puller that I've seen before include "Teasing joker" , "Practical joker" , "Hoaxer" .)