Eventually march back in perhaps? (7)
I believe the answer is:
someday
'eventually' is the definition.
(I've seen this before)
'march back in perhaps?' is the wordplay.
'march' becomes 'demo' (as in a demonstration).
'back' says the letters should be written in reverse.
'in' means one lot of letters goes inside another.
'perhaps?' becomes 'say' (both can mean 'for example').
'demo' back-to-front is 'omed'.
'omed' put inside 'say' is 'SOMEDAY'.
(Other definitions for someday that I've seen before include "eventually" , "song" , "An unspecified time in the future" , "At a future time; samoyed (anag.)" , "When will my prince come" .)