Spring day, content in best suit (6)
I believe the answer is:
spades
'suit' is the definition.
(playing card suit)
'spring day content in best' is the wordplay.
'spring' becomes 'spa' (spa can mean a mineral spring).
'day' becomes 'd' (abbreviation for day).
'content in' indicates the central letters.
The central letters of 'best' are 'es'.
'spa'+'d'+'es'='SPADES'
(Other definitions for spades that I've seen before include "Card suit and sturdy shovels" , "Black suit" , "One of two black suits" , "those getting soiled?" , "Shovels or playing cards" .)