Orbiter going round Mars, perhaps carrying oxygen tank (4,2,2,4)

'tank' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.

'orbiter going round mars perhaps carrying oxygen' is the wordplay.
I cannot really see how this works, but
'orbiter' could be 'comet' (I have seen 'Celestial orbiter ' mean 'comet' so perhaps 'orbiter' could also mean 'comet') and 'comet' is located in the answer.
'going' could be 'on' (both can mean functioning or activated) and 'on' is found in the answer.
'round' could be 'o' (round can mean a round object) and 'o' is located in the answer.
'oxygen' could be 'o' (O is the chemical symbol for oxygen) and 'o' is located in the answer.
'mars perhaps' could be 'god' (mars is a kind of god) and 'god' is present in the leftover letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?