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 understand 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 am not sure about this - if you are sure you should believe this answer much more).
'in' means one lot of letters goes inside another.
'gr' going inside '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" .)