Spring in Regent's Park (3)
I believe the answer is:
spa
'spring' is the definition.
(thesaurus)
'in regent's park' is the wordplay.
'in' says the answer is hidden in the clue.
'SPA' is hidden within 'regents park'.
(Other definitions for spa that I've seen before include "town in Belgium" , "A curative mineral spring, or place with one" , "Health resort near a spring" , "here for the good of his health?" , "Heated bath" .)