Years ago returned for exercise regime (4)
I believe the answer is:
yoga
'exercise regime' is the definition.
(I've seen this before)
'years ago returned' is the wordplay.
'years' becomes 'y' (abbreviation).
'returned' shows that the letters should be reversed in order.
'ago' backwards is 'oga'.
'y'+'oga'='YOGA'
'for' acts as a link.
(Other definitions for yoga that I've seen before include "Hindu spiritual system" , "Hindu physical and mental exercises" , "System of (Hindu) exercises for body and mind" , "Type of exercise" , "System involving physical and mental exercises" .)