Upturn beer cask to find a little food (3)
I believe the answer is:
nut
'a little food' is the definition.
(small food item)
'upturn beer cask' is the wordplay.
'upturn' is a reversal indicator (in a down clue, letters go up).
'beer cask' becomes 'tun' (large container for beer).
'tun' in reverse letter order is 'NUT'.
'to find' is the link.
(Other definitions for nut that I've seen before include "Butt" , "eg Pecan" , "Screwball" , "Mechanical fastener" , "Bonce" .)