Thin soil yielding metallic element (4,5)
I believe the answer is:
rare earth
'metallic element' is the definition.
(I've seen this before)
'thin soil' is the wordplay.
'thin' becomes 'rare' (similar in meaning).
'soil' becomes 'earth' (I've seen this before).
'rare'+'earth'='RARE-EARTH'
'yielding' acts as a link.
(Other definitions for rare earth that I've seen before include "Particular element" , "One of a group of 15 hard to distinguish elements - at her rear (anag)" , "discovery in Ytterby?" , "Oxide" , "metallic element" .)