Being clownish means nothing to a shark, say (6)
I believe the answer is:
oafish
'being clownish' is the definition.
('oafish' can be a synonym of 'clownish')
'nothing to a shark say' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'shark say' becomes 'fish' (shark is a kind of fish).
'o'+'a'+'fish'='OAFISH'
'means' acts as a link.
(Other definitions for oafish that I've seen before include "Stupid, boorish" , "Loutish" , "Like a lout" , "crass" , "lacking elegance?" .)