A great character, possibly in panto (4)
I believe the answer is:
ogre
'panto' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'a great character possibly in' is the wordplay.
'a great' becomes 'gr' (abbreviation used in place names).
'character possibly' becomes 'oe' (I can't explain this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'gr' going into 'oe' is 'OGRE'.
Can you help me to learn more?
(Other definitions for ogre that I've seen before include "Monstrous giant" , "Fairy tale giant" , "Has a monstrous role" , "bad-tempered person" , "The giant is some goer" .)