Spring in St James's Park (3)
I believe the answer is:
spa
'spring' is the definition.
(spa can mean a mineral spring)
'in st james's park' is the wordplay.
'in' says the answer is hidden in the clue.
'SPA' is hidden in the letters of 'james park'.
(Other definitions for spa that I've seen before include "eg Bath" , "Health resort near a spring" , "Water-cure town" , "where health benefits are offered" , "town in Belgium" .)