Shortage of food ultimately overwhelming planet (6)
I believe the answer is:
dearth
'shortage' is the definition.
(synonyms)
'food ultimately overwhelming planet' is the wordplay.
'ultimately' says to take the final letters.
'overwhelming' says to put letters next to each other (in a down clue, some letters go over others).
'planet' becomes 'earth' (Earth is an example).
The final letter of 'food' is 'd'.
'd'+'earth'='DEARTH'
'of' is the link.
(Other definitions for dearth that I've seen before include "Shortage, paucity" , "Scarcity - famine" , "Scarcity - lack" , "Hatred (anag) -- shortage" , "Severe lack" .)