One's smart about working — I wish it were true (2,4)
I believe the answer is:
if only
'i wish it were true' is the definition.
'one's smart about working' is the wordplay.
'one' becomes 'i' (Roman numeral).
'smart' becomes 'fly' (fly can informally mean smart or well-dressed).
'about' indicates putting letters inside.
'working' becomes 'on'.
'fly' placed around 'on' is 'fonly'.
'i'+'fonly'='IF ONLY'
(Other definitions for if only that I've seen before include "expression of wish" , "Would that" , "in my dreams" , "I wish that could be provided just after" .)