Lack of wood, ultimately, on our planet (6)
I believe the answer is:
dearth
'lack of' is the definition.
(I've seen this before)
'wood ultimately on our planet' is the wordplay.
'ultimately' suggests the final letters.
'on' says to put letters next to each other.
'our planet' becomes 'earth' (I've seen this before).
The final letter of 'wood' is 'd'.
'd'+'earth'='DEARTH'
(Other definitions for dearth that I've seen before include "Acute insufficiency" , "Scarcity of thread" , "Scarcity - famine" , "Shortage, lack" , "Severe shortage" .)