So conical a shape must be infrequent (10)
I believe the answer is:
occasional
'infrequent' is the definition.
('occasional' can be similar in meaning to 'infrequent')
'so conical a shape' is the wordplay.
'shape' indicates an anagram.
'so'+'conical'+'a'='soconicala'
'soconicala' with letters rearranged gives 'OCCASIONAL'.
'must be' is the link.
(Other definitions for occasional that I've seen before include "table of a sort" , "Rare" , "Occurring at intervals" , "Periodical" , "Occurring infrequently" .)