I turn back behind unknown Hindu ascetic (4)
I believe the answer is:
yogi
'hindu ascetic' is the definition.
(I've seen this before)
'i turn back behind unknown' is the wordplay.
'turn' becomes 'go' (a person's move in a board game**).
'back' says the letters should be written backwards.
'behind' says to put letters next to each other.
'unknown' becomes 'y' (algebra).
'i'+'go'='igo'
'igo' reversed gives 'ogi'.
'ogi' after 'y' is 'YOGI'.
(Other definitions for yogi that I've seen before include "practitioner of physical discipline" , "Practising philosopher" , "teacher of well-being" , "Someone practising certain Hindu disciplines" , "one accustomed to spirits" .)