Tiny amount of broth — a swiz (7)
I believe the answer is:
soupcon
'tiny amount' is the definition.
'broth a swiz' is the wordplay.
'broth' becomes 'soup' (broth is a type of soup).
'a swiz' becomes 'con' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'soup'+'con'='SOUPCON'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for soupcon that I've seen before include "Hint" , "A vary small quantity, just a trace - from France" , "suspicion" , "Small amount of food" , "Tiny quantity" .)