Stray sheep eating nothing (4)
I believe the answer is:
roam
'stray' is the definition.
(I know that cast can be written as roam)
'sheep eating nothing' is the wordplay.
'sheep' becomes 'ram' (male sheep).
'eating' is an insertion indicator.
'nothing' becomes 'O' (O resembles 0).
'ram' placed around 'o' is 'ROAM'.
(Other definitions for roam that I've seen before include "Ramble without purpose" , "Peregrinate" , "Travel widely" , "Walk without a specific destination" , "Meander" .)