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 can't see how they can define each 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 justify this - if you can you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'gr' inserted 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" .)