Second best part that's a laugh (6)
I believe the answer is:
scream
'that's a laugh' is the definition.
(scream is a kind of laugh)
'second best part' is the wordplay.
'second' becomes 's' (common abbreviation).
'best part' becomes 'cream' (I've seen this before).
's'+'cream'='SCREAM'
(Other definitions for scream that I've seen before include "'Sharp, piercing cry (6)'" , "comedian?" , "Assault victim's reaction, possibly" , "Funny person; cry" , "Hilarious character" .)