Sheep eating nothing small wanders (5)
I believe the answer is:
roams
'wanders' is the definition.
('roam' can be a synonym of 'wander')
'sheep eating nothing small' is the wordplay.
'sheep' becomes 'ram' (ram is a kind of sheep).
'eating' is an insertion indicator.
'nothing' becomes 'o' (looks like zero - 0).
'small' becomes 's' (abbreviation - e.g. clothes size).
'ram' placed around 'o' is 'roam'.
'roam'+'s'='ROAMS'
(Other definitions for roams that I've seen before include "Wanders about, rambles" , "Is traveller" .)