Sounds like an opportunity to buy something in Knock (6)
I believe the answer is:
assail
'knock' is the definition.
Although both the answer and definition are verbs in their base form, I can't understand how one could define the other.
'sounds like an opportunity to buy something' is the wordplay.
I cannot quite see how this works, but
'like' could be 'as' and 'as' is found within the answer.
'an' could be 'i' (Roman numeral for one) and 'i' is located in the answer.
The remaining letters 'sal' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'in' is the link.
Can you help me to learn more?
(Other definitions for assail that I've seen before include "Set upon" , "Aggressively attack" , "Assault, attack" , "Sudden attack" , "storm" .)