Unknown in the past - twisted philosophy (4)
I believe the answer is:
yoga
'philosophy' is the definition.
(I have seen 'The philosophy' mean 'yoga' so perhaps 'philosophy' could also mean 'yoga')
'unknown in the past twisted' is the wordplay.
'unknown' becomes 'y' (algebra).
'in the past' becomes 'ago' (synonyms).
'twisted' says the letters should be written in reverse.
'ago' written backwards gives 'oga'.
'y'+'oga'='YOGA'
(Other definitions for yoga that I've seen before include "(Hindu) mind and body toning" , "Hindu system for health and relaxation" , "Hindu spiritual system" , "Posturing" , "Healthy exercises" .)