Understand everything about wise guy (4-3)
I believe the answer is:
know-all
'wise guy' is the definition.
(I've seen this before)
'understand everything' is the wordplay.
'understand' becomes 'know' (I've seen this before).
'everything' becomes 'all' (all things).
'know'+'all'='KNOW-ALL'
'about' acts as a link.
(Other definitions for know-all that I've seen before include "Smart alec" , "Smarty-pants" , "Wise guy" , "One who thinks they have all the answers" , "Self-styled polymath" .)