Finishes pastry - but not all at once (4-5)

'once' is the definition.
I don't know anything about this answer so I cannot judge whether this works.

'finishes pastry but not all' is the wordplay.
'finishes' becomes 'stops' ('stop' can be a synonym of 'finish').
'pastry' becomes 'tarts' (tart is a type of pastry product).
'but not all' means to remove the last letter.
'tarts' with its final letter removed is 'tart'.
'stops'+'tart'='STOP-START'

'at' acts as a link.

Can you help me to learn more?