Small prune that’s fed to pigs (4)
I believe the answer is:
slop
'that's fed to pigs' is the definition.
(liquid fed to animals)
'small prune' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'prune' becomes 'lop' (synonyms).
's'+'lop'='SLOP'
(Other definitions for slop that I've seen before include "(Of liquid) spill over" , "Cause to spill over" , "Splash" , "Liquid refuse, swill" , "Spilled liquid" .)