Second testament is hogwash (5)
I believe the answer is:
swill
'is hogwash' is the definition.
(I've seen this before)
'second testament' is the wordplay.
'second' becomes 's' (common abbreviation).
'testament' becomes 'will' (synonyms).
's'+'will'='SWILL'
(Other definitions for swill that I've seen before include "Hogwash" , "Waste food left in wills" , "Wet feed for pigs" , "drink heavily" , "Wash" .)