Sun's 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)
'sun's best part?' is the wordplay.
'sun' becomes 's'.
'best part?' becomes 'cream' (I've seen this before).
's'+'cream'='SCREAM'
(Other definitions for scream that I've seen before include "Extremely funny entertainer" , "funny man" , "card" , "Cry of anguish" , "comedian?" .)