Spiked over, it’s on volume (9)
I believe the answer is:
decalitre
'volume' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'spiked over it's on' is the wordplay.
'spiked' becomes 'laced' ('lace' can be a synonym of 'spike').
'over' says the letters should be written in reverse.
'it's' becomes 'it' (I've seen this in other clues).
'on' becomes 're' ('on' can be mean 'with reference to').
'laced' reversed gives 'decal'.
'decal'+'it'+'re'='DECALITRE'
Can you help me to learn more?
(Other definitions for decalitre that I've seen before include "Quantity of liquid" , "several pints" .)