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 tell 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 am not sure about this - if you are sure you should believe this answer much more).
'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" .)