After a time, alter rashly! (5)
I believe the answer is:
later
'after a time' is the definition.
(I've seen this in another clue)
'alter rashly' is the wordplay.
'rashly' indicates anagramming the letters.
'alter' with letters rearranged gives 'LATER'.
(Other definitions for later that I've seen before include "At some time afterwards" , "succeeding" , "Alert (anag.)" , "At a time in the future" , "goodbye" .)