It's mysterious. Red Cross takes in a seat of learning (5)
I believe the answer is:
runic
'it's mysterious' is the definition.
(I've seen this in another clue)
'red cross takes in a seat of learning' is the wordplay.
'red cross' becomes 'rc'.
'takes in' means one lot of letters goes inside another.
'a seat of learning' becomes 'uni' (I've seen this before).
'rc' enclosing 'uni' is 'RUNIC'.
(Other definitions for runic that I've seen before include "Relating to old Germanic alphabet" , "character hard to interpret" , "Using Norse letters" , "Like mysterious characters" , "Inscribed with ancient symbols" .)