First of seasons each year is spring (3)
I believe the answer is:
spa
'is spring' is the definition.
(thesaurus)
'first of seasons each year' is the wordplay.
'first of' suggests taking the first letters.
'each year' becomes 'pa' ('p.a.' can be a synonym of 'each year').
The initial letter of 'seasons' is 's'.
's'+'pa'='SPA'
(Other definitions for spa that I've seen before include "Health resort near a spring" , "Watering hole" , "well" , "place of treatment" , "Health and beauty treatment establishment" .)