Buy a three-pound mix that soothes infant (7)
I believe the answer is:
lullaby
'mix that soothes infant' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'buy a three-pound' is the wordplay.
I cannot really see how this works, but
'a' is within the answer.
'pound' could be 'lb' (abbreviation for the unit of weight) and 'lb' is located in the leftover letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for lullaby that I've seen before include "Gentle song to put baby to sleep" , "Soothing song for infants" , "Go-to-sleep song" , "Soothing song for child" , "Sleep-inducing song" .)