It's used when serving starter of tinned fish (4)
I believe the answer is:
tray
'it's used when' is the definition.
I can't judge whether this defines the answer.
'starter of tinned fish' is the wordplay.
'starter of' suggests taking the first letters.
'fish' becomes 'ray' (ray is a kind of fish).
The first letter of 'tinned' is 't'.
't'+'ray'='TRAY'
'serving' acts as a link.
Can you help me to learn more?
(Other definitions for tray that I've seen before include "Item used for carrying food and drink" , "Shallow receptable for carrying something" , "Flat plate for carrying things" , "Informal meal" , "Flat carrier" .)