Mars? Admit flying to Mars is to be a success (2,4,1,5)
I believe the answer is:
go down a storm
'mars? admit' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'flying to mars is to be a success' is the wordplay.
'flying' indicates an anagram (letters fly around into new positions).
'is to' means one lot of letters go next to another (I've seen this in other clues).
'be a success' becomes 'godown' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to'+'mars'='tomars'
'tomars' with letters rearranged gives 'astorm'.
'astorm' put after 'godown' is 'GO DOWN A STORM'.
Can you help me to learn more?
(Another definition for go down a storm that I've seen is " get great reception".)