Observation about evangelist (6)
I believe the answer is:
remark
'observation' is the definition.
(I know that remark is a type of observation)
'about evangelist' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'evangelist' becomes 'Mark' (St Mark).
're'+'mark'='REMARK'
(Other definitions for remark that I've seen before include "Incidental observation" , "Make mention of" , "Written or spoken comment" , "Something said; take notice of" , "Observation - notice" .)