Thus the advertisement is put up for chemical substance (4)
I believe the answer is:
soda
'chemical substance' is the definition.
'soda' can be an answer for 'substance' (soda is a kind of substance). I'm unsure of the 'chemical' bit.
'thus the advertisement is put up' is the wordplay.
'thus' becomes 'so' (synonyms).
'the advertisement' becomes 'ad' (short for advert).
'is put up' shows that the letters should be reversed in order.
'ad' back-to-front is 'da'.
'so'+'da'='SODA'
'for' is the link.
Can you help me to learn more?
(Other definitions for soda that I've seen before include "In the US, a sweet fizzy drink" , "Fizzy water, goes well with whiskey" , "Sodium carbonate" , "Variety of pop" , "Mixer drink" .)