Popular girl's set out of sorts (10)
I believe the answer is:
indisposed
'out of sorts' is the definition.
(I've seen this before)
'popular girl's set' is the wordplay.
'popular' becomes 'in' (something that's 'in' is fashionable).
'girl's' becomes 'Dis' (short for Diane or Diana).
'set' becomes 'posed'.
'in'+'dis'+'posed'='INDISPOSED'
(Other definitions for indisposed that I've seen before include "Poorly" , "Out of sorts" , "Slightly unwell" .)