Relative turned about to go for soup (5)
I believe the answer is:
broth
'soup' is the definition.
(broth is a kind of soup)
'relative turned about to go' is the wordplay.
'relative' becomes 'bro' (short for brother).
'turned' shows that the letters should be reversed in order.
'about to go' becomes 'ht' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ht' backwards is 'th'.
'bro'+'th'='BROTH'
'for' is the link.
Can you help me to learn more?
(Other definitions for broth that I've seen before include "Kind of soup" , "Setting for culture" , "Type of soup" , "Bouillon" , "Stock" .)