Cat's contented sound is endless, provided you firstly decontaminate it (6)
I believe the answer is:
purify
'decontaminate it' is the definition.
I can't tell whether this definition defines the answer.
'cat's contented sound is endless provided you firstly' is the wordplay.
'cat's contented sound' becomes 'purr' (I've seen this before).
'is endless' means to remove the last letter.
'provided' becomes 'if' (eg 'provided it's sunny' means 'if it's sunny').
'firstly' says to take the initial letters.
The initial letter of 'you' is 'y'.
'purr' with its last letter taken off is 'pur'.
'pur'+'if'+'y'='PURIFY'
Can you help me to learn more?
(Other definitions for purify that I've seen before include "Free from guilt" , "Decontaminate" , "Cleanse" , "Clean" , "Remove contamination from" .)