Tea: a large quantity from the East (5)
I believe the answer is:
assam
'tea' is the definition.
(I've seen this before)
'a large quantity from the east' is the wordplay.
'large quantity' becomes 'mass' (mass is a kind of quantity. I am not sure about the 'large' bit.).
'from the east' is a reversal indicator.
'mass' reversed gives 'ssam'.
'a'+'ssam'='ASSAM'
Can you help me to learn more?
(Other definitions for assam that I've seen before include "Leaves to get in hot water" , "Indian state, famed for its tea" , "Variety of tea" , "brew?" , "Tea-growing state in north-east India" .)