After a time, alter rashly! (5)
I believe the answer is:
later
'after a time' is the definition.
(I've seen this before)
'alter rashly' is the wordplay.
'rashly' indicates an anagram.
'alter' anagrammed 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" .)