Spray cap with inner spring (9)
I believe the answer is:
bespatter
'spray' is the definition.
(thesaurus)
'cap with inner spring' is the wordplay.
'cap' becomes 'better' (I've seen this before).
'with inner' indicates putting letters inside.
'spring' becomes 'spa' (spa can mean a mineral spring).
'better' enclosing 'spa' is 'BESPATTER'.
(Other definitions for bespatter that I've seen before include "Defame" , "Spray with droplets" , "Make mess of" , "Dirty" .)