One or two items on the fishmonger’s slab (6,5)

'fishmonger's slab' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

'one or two items on the' is the wordplay.
I cannot really understand how this works, but
'one' could be 'a' (a thing is one thing) and 'a' is located in the answer.
'on' is present in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
This explanation may well be incorrect...

Can you help me to learn more?