Popular courtiers mixed up with nits rebel (15)
I believe the answer is:
insurrectionist
'rebel' is the definition.
(I know that rebel can be written as insurrectionist)
'popular courtiers mixed up with nits' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'mixed up with' is an anagram indicator.
'courtiers' with its letters anagrammed with 'nits' can make 'surrectionist'.
'in'+'surrectionist'='INSURRECTIONIST'
(Other definitions for insurrectionist that I've seen before include "Revolting type" , "One giving revolutionary" .)