Large old vessel, a muted green (5)
I believe the answer is:
lovat
'a muted green' is the definition.
'lovat' can be an answer for 'green' (I've seen this before). I'm not certain of the 'a muted' bit.
'large old vessel' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'vessel' becomes 'vat' (vat is a kind of vessel).
'l'+'o'+'vat'='LOVAT'
Can you help me to learn more?
(Other definitions for lovat that I've seen before include "(Of woollen cloth) muted green-blue colour" , "Green cloth" , "sort of green" , "Green colour, as of tweed" , "Bluish-green tweed" .)