Small swine turned to tap on barrel (6)
I believe the answer is:
spigot
'tap on barrel' is the definition.
(I've seen this before)
'small swine turned to' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'swine' becomes 'pig' (pig is a kind of swine).
'turned' says the letters should be written in reverse.
'to' back-to-front is 'ot'.
's'+'pig'+'ot'='SPIGOT'
(Other definitions for spigot that I've seen before include "Peg that stops the vent of a cask" , "Wooden tap on a cask" , "Peg - plug - tap" , "Plug for a cask's vent hole" , "Bunghole plug" .)