Greek letter hidden in vase for waif (6)
I believe the answer is:
urchin
'waif' is the definition.
(I've seen this before)
'greek letter hidden in vase' is the wordplay.
'greek letter' becomes 'chi' (letter from the Greek alphabet).
'hidden in' means one lot of letters goes inside another.
'vase' becomes 'urn' (both can mean a container).
'chi' going inside 'urn' is 'URCHIN'.
'for' is the link.
(Other definitions for urchin that I've seen before include "Ragged child or sea creature with round shell" , "Hedgehog - ragamuffin" , "Ragged child; marine animal" , "Sea creature or mischievous child" , "Sea creature with a spiny shell" .)