Cost singular to get to Mars (7)
I believe the answer is:
damages
'mars' is the definition.
(to mar is to damage or disfigure)
'cost singular' is the wordplay.
'cost' becomes 'damage' (damage is a kind of cost).
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'damage'+'s'='DAMAGES'
'to get to' acts as a link.
(Other definitions for damages that I've seen before include "Money awarded as compensation" , "weakens" , "Hurts" , "Moneys paid as compensation for an injury" , "Harms" .)