Diamonds on ground in shortage (6)
I believe the answer is:
'shortage' is the definition.
(dearth can mean a shortage or lack)
'diamonds on ground' is the wordplay.
'diamonds' becomes 'd' (abbreviation for the card suit diamonds).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'ground' becomes 'earth' (synonyms).
'in' acts as a link.
(Other definitions for dearth that I've seen before include "Hatred (anag) -- shortage" , "Shortage, lack" , "From shortage of food" , "Scarceness or lack" , "Severe shortage" .)