Most clownish enthusiasm about a black bird (7)
I believe the answer is:
zaniest
'most clownish' is the definition.
('zany' can be a synonym of 'clownish')
'enthusiasm about a black bird' is the wordplay.
'enthusiasm' becomes 'zest' (zest can mean enthusiasm or eagerness).
'about' means one lot of letters goes inside another.
'a black bird' becomes 'ani' (I've seen this in another clue).
'zest' going around 'ani' is 'ZANIEST'.
(Other definitions for zaniest that I've seen before include "ludicrous as can be" , "Most crackers" , "Most crazy" , "most bizarre" , "[OPPOSITE OF] most normal" .)