Look up "subsistence" (4)
I believe the answer is:
keep
'subsistence' is the definition.
(I've seen this before)
'look up' is the wordplay.
'look' becomes 'peek' (peek is a kind of look).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'peek' back-to-front is 'KEEP'.
(Other definitions for keep that I've seen before include "Board and lodging" , "Eg, beef" , "strong position" , "Castle tower; cost of maintenance" , "Save" .)