Knowing when to find time in day (6)
I believe the answer is:
astute
'knowing' is the definition.
(thesaurus)
'when to find time in day' is the wordplay.
'when' becomes 'as'.
'to find' says to put letters next to each other.
'time' becomes 't'.
'in' indicates putting letters inside.
'day' becomes 'tue' (**).
't' placed inside 'tue' is 'tute'.
'as'+'tute'='ASTUTE'
(Other definitions for astute that I've seen before include "Clever and crafty" , "Perspicacious" , "Peerceptive" , "Wide" , "Shrewd, canny" .)