Popular story about sick revolutionary is perfect (10)
I believe the answer is:
infallible
'is perfect' is the definition.
(I've seen this before)
'popular story about sick revolutionary' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'story' becomes 'fable' (fable is a kind of story).
'about' means one lot of letters goes inside another.
'sick' becomes 'ill' (synonyms).
'revolutionary' says the letters should be written backwards.
'ill' back-to-front is 'lli'.
'fable' placed around 'lli' is 'fallible'.
'in'+'fallible'='INFALLIBLE'
(Other definitions for infallible that I've seen before include "faultless" , "Perfect" , "Always right" , "Unable to make a mistake" , "Incapable of error" .)