Drifts round, hemmed in by sheep (5)
I believe the answer is:
Here is my best explanation:
'drifts' is the definition.
('roam' can be a synonym of 'drift')
'round hemmed in by sheep' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'hemmed in by' means one lot of letters goes inside another.
'sheep' becomes 'rams' (I have seen 'Male sheep ' mean 'rams' so perhaps 'sheep' could also mean 'rams').
'o' placed within 'rams' is 'ROAMS'.
(Other definitions for roams that I've seen before include "Wanders about, rambles", "wanders about aimlessly" and "Is traveller".)