In which 'essence' is an intellectual pursuit (5)
I believe the answer is:
chess
'an intellectual pursuit' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'in which essence is' is the wordplay.
'in' says the answer is hidden in the clue.
'CHESS' is hidden within 'which essence is'.
Can you help me to learn more?
(Other definitions for chess that I've seen before include "Board game for two players" , "A game of skill" , "In which two bishops will participate" , "activity aimed at downfall of a king" , "Sixteen-men-a-side game" .)