Thus, on some days, it's used for washing (4)
I believe the answer is:
soda
'used for washing' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'thus on some days' is the wordplay.
'thus' becomes 'so' (synonyms).
'on' means one lot of letters go next to another.
'some' becomes 'a' ().
'days' becomes 'd' (abbreviation for days).
'a' after 'd' is 'da'.
'so'+'da'='SODA'
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for soda that I've seen before include "kind of water" , "Soft drink" , "for use with whisky?" , "Fizzy drink - chemical compound" , "In the US, a sweet fizzy drink" .)