Fictional character Mighty Mouse keeps heading skyward (4)
I believe the answer is:
myth
'fictional character' is the definition.
(I know that myth is a type of fictional character)
'mighty mouse keeps heading skyward' is the wordplay.
'keeps' indicates a hidden word.
'heading skyward' says the letters should be written backwards (in a down clue, heading 'up').
'htym' can be found hidden inside 'mighty mouse'.
'htym' back-to-front is 'MYTH'.
(Other definitions for myth that I've seen before include "Fictitious thing; traditional story" , "Traditional story not based on fact" , "A widely held but false belief" , "Legend, folk tale" , "false interpretation?" .)