Complaint of one in fancy summer hat (10)
I believe the answer is:
rheumatism
'complaint' is the definition.
(rheumatism is a kind of disease)
'one in fancy summer hat' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'fancy' is an anagram indicator.
'summer'+'hat'='summerhat'
'summerhat' with letters rearranged gives 'rheumatsm'.
'i' placed into 'rheumatsm' is 'RHEUMATISM'.
'of' is the link.
(Other definitions for rheumatism that I've seen before include "Inflammation of the joints" , "ailment" , "Joint affliction" .)