Fashionable diamonds one's set poorly (10)
I believe the answer is:
indisposed
'poorly' is the definition.
(I know that poorly can be written as indisposed)
'fashionable diamonds one's set' is the wordplay.
'fashionable' becomes 'in' (in can mean fashionable or trendy).
'diamonds' becomes 'd' (abbreviation in bridge say).
'one's' becomes 'i's' (Roman numeral).
'set' becomes 'posed' ().
'in'+'d'+'is'+'posed'='INDISPOSED'
(Other definitions for indisposed that I've seen before include "Slightly unwell" , "Poorly" , "Out of sorts" .)