Tea service one knocked over (5)
I believe the answer is:
assam
'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.
'mass'+'a'='massa'
'massa' backwards is 'ASSAM'.
(Other definitions for assam that I've seen before include "North-east Indian tea state" , "region out east" , "Tea-growing region" , "part of India" , "Variety of tea" .)