Observation on one of the gospels
I believe the answer is:
remark
'observation' is the definition.
(remark is a kind of observation)
'on one of the gospels' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'one of the gospels' becomes 'mark' (I can't explain this - if you can you should give a lot more credence to this answer).
're'+'mark'='REMARK'
Can you help me to learn more?
(Other definitions for remark that I've seen before include "Something said; take notice of" , "Written or spoken comment" , "Comment, observation" , "Say as a comment" , "Comment or mention" .)