That extra spring again! (5)
I believe the answer is:
spare
'that extra' is the definition.
(both can mean left over)
'spring again' is the wordplay.
'spring' becomes 'spa' (spa can mean a mineral spring).
'again' becomes 're'.
'spa'+'re'='SPARE'
(Other definitions for spare that I've seen before include "This tyre around the middle is fat" , "Additional - for use when needed" , "Excess to requirements" , "A feature of trousers" , "slit at top of trousers in Scotland" .)