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 can mean to look at quickly).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'peek' reversed gives 'KEEP'.
(Other definitions for keep that I've seen before include "Board and lodging" , "Eg, beef" , "strong position" , "Castle tower; cost of maintenance" , "Save" .)