Good African currency’s ready for younger relative (8)
I believe the answer is:
Here is my best explanation:
'younger relative' is the definition.
'good african currency's ready' is the subsidiary indication.
'good' becomes 'g' (abbreviation).
'african currency' becomes 'rand'.
'ready' becomes 'son' (I can't explain this - if you can you should believe this answer much more).
'for' is the link.
(Other definitions for grandson that I've seen before include "Relative", "Male descendant", "Descendant", "Dons rang (anag) - relative", "Male offspring, third generation (8)", "Male relative", "Young male relative", "Male child of one's child", "Rang Don's relative" and "Daughter's boy".)