Tsar can earn mostly in secret (6)
I believe the answer is:
arcane
'secret' is the definition.
(I've seen this in another clue)
'tsar can earn mostly in' is the wordplay.
'in' indicates a hidden word.
'ARCANE' is hidden in the letters of 'tsar can earn mostly'.
(Other definitions for arcane that I've seen before include "Mysterious, secret or obscure" , "Requiring secret or mysterious knowledge" , "Ace ran, being mysterious" , "Abstruse, mysterious" , "deep" .)