Lack of wood, ultimately, on planet (6)
I believe the answer is:
dearth
'lack of' is the definition.
(I've seen this before)
'wood ultimately on planet' is the wordplay.
'ultimately' says to take the final letters.
'on' means one lot of letters go next to another.
'planet' becomes 'earth' (Earth is an example).
The last letter of 'wood' is 'd'.
'd'+'earth'='DEARTH'
(Other definitions for dearth that I've seen before include "Scarcity - lack" , "Acute shortage" , "Shortage, lack" , "Severe shortage" , "Hatred (anag) -- shortage" .)