Health resort son has joined annually (3)
I believe the answer is:
spa
'health resort' is the definition.
(thesaurus)
'son has joined annually' is the wordplay.
'son' becomes 's'.
'has' is a charade indicator (letters next to each other).
'joined annually' becomes 'pa' (I can't explain this - if you can you should give a lot more credence to this answer).
's'+'pa'='SPA'
Can you help me to learn more?
(Other definitions for spa that I've seen before include "town in Belgium" , "Boston" , "here for the good of his health?" , "Health resort at a mineral spring" , "A therapeutic hot spring" .)