It's hard to say to utter swine, out drinking gallons (6-7)
I believe the answer is:
tongue twister
'it's hard to say' is the definition.
(I've seen this in another clue)
'to utter swine out drinking gallons' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'drinking' is an insertion indicator (inserted letters are consumed or drunk).
'gallons' becomes 'G' (abbreviation).
'to'+'utter'+'swine'='toutterswine'
'toutterswine' is an anagram of 'tonuetwister'.
'tonuetwister' placed around 'g' is 'TONGUE-TWISTER'.
(Other definitions for tongue twister that I've seen before include "Difficult saying?" , "Hard-to-say phrase" , "Test of verbal dexterity" , "Peter Piper picked a peck of pickled pepper, for instance" , "'I wish to wash my Irish wristwatch', perhaps" .)