Frank's into Schopenhauer (4)
I believe the answer is:
open
'frank's' is the definition.
(synonyms)
'into schopenhauer' is the wordplay.
'into' indicates the answer is hidden within the clue.
'OPEN' is hidden within 'schopenhauer'.
(Other definitions for open that I've seen before include "such a verdict?" , "golf contest" , "Traditional room heater" , "With which to offer hearty welcome" , "Unlock" .)