Knowing when to express disapproval on a point (6)
I believe the answer is:
astute
'knowing' is the definition.
(thesaurus)
'to express disapproval on a point' is the wordplay.
'to express disapproval' becomes 'tut' (I've seen this in other clues).
'on' is an insertion indicator.
'point' becomes 'se' (SE is an example**).
'a'+'se'='ase'
'tut' put within 'ase' is 'ASTUTE'.
'when' acts as a link.
(Other definitions for astute that I've seen before include "Smart and canny" , "Shrewd, canny" , "'Perceptive, shrewd (6)'" , "Shrewd and clever" , "on the ball" .)