Methane planet has -- breathing ultimately 'restricted'! (5-3)
I believe the answer is:
marsh gas
'methane' is the definition.
(marsh gas is a kind of methane)
'planet has breathing ultimately restricted' is the wordplay.
'planet' becomes 'mars' (Mars is a planet).
'ultimately' says to take the final letters.
'restricted' indicates putting letters inside.
The last letter of 'breathing' is 'g'.
'has' placed around 'g' is 'hgas'.
'mars'+'hgas'='MARSH-GAS'
(Other definitions for marsh gas that I've seen before include "Methane from decaying matter in bogs" , "Product of rotting waterlogged matter" , "methane type" .)