The only part of the Trent used by yachts (6)
I believe the answer is:
solent
'trent used by yachts' is the definition.
I can't judge whether this defines the answer.
'the only part of the' is the wordplay.
'the only' becomes 'sole' (I've seen this before).
'part of' becomes 'n' (I can't explain this - if you can you should give a lot more credence to this answer).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'sole'+'n'+'t'='SOLENT'
Can you help me to learn more?
(Other definitions for solent that I've seen before include "Channel north of the Isle of Wight" , "stretch of water" , "Water to north of IOW" , "body of water" , "Strait off Portsmouth" .)