Drifts round, hemmed in by sheep (5)
I believe the answer is:
roams
'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" , "Is traveller" .)