Prevented from being a drifter (6)
I believe the answer is:
moored
'prevented from being a drifter' is the definition.
I can't tell whether this definition defines the answer.
'prevented from being' is the wordplay.
'prevented from' becomes 'moo' (I can't explain this - if you can you should give a lot more credence to this answer).
'being' becomes 'red' (red is a kind of being).
'moo'+'red'='MOORED'
Can you help me to learn more?
(Other definitions for moored that I've seen before include "Tied up to berth like ship or boat" , "Berthed, tied up" , "Secured by rope or cable" , "Of boat, tied up or secured" , "Made fast" .)