Cleaning agent very suitable when scratching bottom (4)
I believe the answer is:
soap
'cleaning agent' is the definition.
(I've seen this before)
'very suitable when scratching bottom' is the wordplay.
'very' becomes 'so' (both can mean 'extremely').
'suitable' becomes 'apt' (I've seen this before).
'when scratching bottom' means to remove the last letter (remove bottom letter in down clue).
'apt' with its final letter taken off is 'ap'.
'so'+'ap'='SOAP'
(Other definitions for soap that I've seen before include "It has detergent properties" , "Cleansing agent" , "TV series of cleansing sort" , "Serialised drama" , "TV programme; wash" .)