Witness mates, hammered, opening old wine (9)
I believe the answer is:
testament
'witness' is the definition.
(I've seen this before)
'mates hammered opening old wine' is the wordplay.
'hammered' is an anagram indicator (slang term for drunk).
'opening' is an insertion indicator (some letters open a gap in others).
'old wine' becomes 'tent' (type of Spanish wine. I am not sure about the 'old' bit.).
'mates' is an anagram of 'estam'.
'estam' placed within 'tent' is 'TESTAMENT'.
Can you help me to learn more?
(Other definitions for testament that I've seen before include "Will or solemn declaration" , "Tribute" , "New and old parts of the Bible" , "Witness" , "Earnest" .)