Thus, on an annual occasion, it's used for yachting (6)
I believe the answer is:
solent
'for yachting' is the definition.
The definition suggests a singular noun which matches the answer.
'thus on an annual occasion it's' is the wordplay.
'thus' becomes 'so' (synonyms).
'on' means one lot of letters go next to another.
'an annual occasion' becomes 'len' (I can't explain this - if you can you should believe this answer much more).
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'so'+'len'+'t'='SOLENT'
'used' acts as a link.
Can you help me to learn more?
(Other definitions for solent that I've seen before include "body of water" , "Channel north of the Isle of Wight" , "sound" , "Channel north of IOW" , "The ---, water N. of I. of Wight" .)