Belgium, in no small amount, is pretty good! (3,3)
I believe the answer is:
not bad
'pretty good' is the definition.
(I've seen this in another clue)
'belgium in no small amount' is the wordplay.
'belgium' becomes 'B' (international vehicle registration code).
'in' indicates putting letters inside.
'small amount' becomes 'tad' ('a tad' is a small amount).
'no'+'tad'='notad'
'b' inserted into 'notad' is 'NOT BAD'.
'is' is the link.
(Other definitions for not bad that I've seen before include "Fairly good, all things considered" , "Informally good, a bit grudging" , "fine" , "could be OK" , "Acceptable" .)