Rare thing our don then replaced (4,2,3,6)
I believe the answer is:
thin on the ground
'rare' is the definition.
(I've seen this before)
'thing our don then replaced' is the wordplay.
'replaced' is an anagram indicator.
'thing'+'our'+'don'+'then'='thingourdonthen'
'thingourdonthen' anagrammed gives 'THIN ON THE GROUND'.
(Other definitions for thin on the ground that I've seen before include "Rare" , "A sprinkling of snow" , "description of weedy fighter after KO?" , "scarce" , "Present in very small numbers" .)