Accountant set out agreement for gems (4-4)
I believe the answer is:
cats-eyes
'gems' is the definition.
(cat's eye is a kind of gem)
'accountant set out agreement' is the wordplay.
'accountant' becomes 'CA' (abbreviation for chartered accountant).
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'agreement' becomes 'yes' (saying yes to something).
'set' with letters rearranged gives 'tse'.
'ca'+'tse'+'yes'='CATS-EYES'
'for' is the link.
(Other definitions for cats-eyes that I've seen before include "On the road, they reflect light" , "Road safety devices" , "Road reflectors to guide traffic, of feline origin" , "Line of reflective studs as road markings- very feline" , "Middle-of-the-road reflectors" .)