Choose part of film containing limitless vice — it's a shocker! (8,3)
I believe the answer is:
electric eel
'a shocker' is the definition.
(I've seen this before)
'choose part of film containing limitless vice' is the wordplay.
'choose' becomes 'elect' (electing is a kind of choosing).
'part of film' becomes 'reel' (as in a reel of film).
'containing' is an insertion indicator.
'limitless' indicates the central letters.
The centre of 'vice' is 'ic'.
'reel' going around 'ic' is 'riceel'.
'elect'+'riceel'='ELECTRIC EEL'
'it's' is the link.
(Other definitions for electric eel that I've seen before include "shocker" , "Shocking fish" .)