Smart alec with different guise by the end of the day (4,3)
I believe the answer is:
wise guy
'smart alec' is the definition.
(I've seen this before)
'with different guise by the end of the day' is the wordplay.
'with' becomes 'w' (short for with).
'different' is an anagram indicator (letters placed in a different order).
'by' means one lot of letters go next to another.
'the end of the' suggests the final letters.
The last letter of 'day' is 'y'.
'guise' is an anagram of 'isegu'.
'w'+'isegu'+'y'='WISE GUY'
(Other definitions for wise guy that I've seen before include "smart Aleck" , "Smart alec (US)" .)