Tea service one knocked over (5)
I believe the answer is:
Here is my best explanation:
'tea' is the definition.
(I've seen this before)
'service one knocked over' is the wordplay.
'service' becomes 'mass' (I've seen this in other clues).
'one' becomes 'a'.
'knocked over' is a reversal indicator.
'massa' backwards is 'ASSAM'.
(Other definitions for assam that I've seen before include "Indian tea-growing area", "Tea state of India", "part of India", "Sort of tea", "Kind of Indian tea", "Tea-growing Indian state", "Leaves to get in hot water", "Top quality tea", "North-east Indian tea state" and "Quality cuppa".)