Famine for a large number on this planet (6)
I believe the answer is:
dearth
'famine' is the definition.
(a famine is a dearth of food)
'large number on this planet' is the wordplay.
'large number' becomes 'd'.
'on' says to put letters next to each other.
'this planet' becomes 'earth' (Earth is an example).
'd'+'earth'='DEARTH'
'for a' is the link.
(Other definitions for dearth that I've seen before include "Severe shortage" , "From shortage of food" , "Shortage, paucity" , "deficiency" , "Red hat for scarcity" .)