It's crazy, going up over a long time for Mars (7)
I believe the answer is:
damages
'for mars' is the definition.
(to mar is to damage or disfigure)
'it's crazy going up over a long time' is the wordplay.
'it's crazy' becomes 'mad' (synonyms).
'going up' says the letters should be written backwards (in a down clue, the letters are written upwards).
'over' says to put letters next to each other (in a down clue, some letters go over others).
'a long time' becomes 'ages' (ages can mean a very long time).
'mad' written backwards gives 'dam'.
'dam'+'ages'='DAMAGES'
(Other definitions for damages that I've seen before include "Hurts" , "Harms" , "weakens" , "Money paid in compensation for loss or injury" , "Moneys paid as compensation for an injury" .)