Planet faced with onset of deadly shortage? (6)
I believe the answer is:
dearth
'shortage?' is the definition.
('dearth' can be a synonym of 'shortage')
'planet faced with onset of deadly' is the wordplay.
'planet' becomes 'earth' (Earth is an example).
'faced with' means one lot of letters go next to another.
'onset of' suggests taking the first letters.
The initial letter of 'deadly' is 'd'.
'earth' after 'd' is 'DEARTH'.
(Other definitions for dearth that I've seen before include "Shortage, lack" , "Scarcity - lack" , "Insufficient amount" , "Scarcity - famine" , "Hatred (anag) -- shortage" .)