Bit of fish finger, half gone (3)
I believe the answer is:
fin
'bit of fish' is the definition.
(fish have fins)
'finger half gone' is the wordplay.
'half gone' suggests halving the letters.
'finger' with half the letters taken is 'FIN'.
(Other definitions for fin that I've seen before include "Paddle-like organ" , "Part of an aircraft's tail" , "Thin projection for fish to steer itself" , "Fishy appendage" , "Piscine appendage" .)