Magpie in greyish-white, colour seen from below (7)
I believe the answer is:
hoarder
'magpie' is the definition.
(magpie can mean someone who hoards things)
'greyish-white colour seen from below' is the wordplay.
'greyish-white' becomes 'hoar' (hoar means greyish-white in colour).
'colour' becomes 'red' (red is a kind of colour).
'seen from below' says the letters should be written backwards (in a down clue, letters as seen from below).
'red' written backwards gives 'der'.
'hoar'+'der'='HOARDER'
'in' is the link.
(Other definitions for hoarder that I've seen before include "Person acting like a squirrel" , "Accumulator" , "18 [MAGPIE] , perhaps" , "One with hidden store" , "One who accumulates or stockpiles" .)