With the Sunday best on, is really funny (6)
I believe the answer is:
scream
'is really funny' is the definition.
I can't judge whether this defines the answer.
'with the sunday best' is the wordplay.
'with' means one lot of letters go next to another.
'the sunday' becomes 'S'.
'best' becomes 'cream' (both can mean the elite).
's' put next to 'cream' is 'SCREAM'.
'on' is the link.
Can you help me to learn more?
(Other definitions for scream that I've seen before include "one causes great amusement" , "'Sharp, piercing cry (6)'" , "Howl, shriek" , "Assault victim's reaction, possibly" , "Scar me (anag)" .)