Condition of one wearing new summer hat? (10)
I believe the answer is:
rheumatism
'condition' is the definition.
(medical condition)
'one wearing new summer hat?' is the wordplay.
'one' becomes 'i' (roman numeral).
'wearing' indicates putting letters inside.
'new' indicates an anagram.
'summer'+'hat'='summerhat'
'summerhat' anagrammed gives 'rheumatsm'.
'i' inserted within 'rheumatsm' is 'RHEUMATISM'.
'of' acts as a link.
(Other definitions for rheumatism that I've seen before include "ailment" , "Joint affliction" , "Inflammation of the joints" .)