Express disapproval to that man's son (4)
I believe the answer is:
hiss
'express disapproval' is the definition.
(I've seen this in another clue)
'that man's son' is the wordplay.
'that man' becomes 'hi' ().
'son' becomes 'ss' (I've seen this in other clues).
'hi'+'ss'='HISS'
'to' acts as a link.
(Other definitions for hiss that I've seen before include "less welcome sign?" , "S-sound of disapproval" , "Sibilant sound made through the teeth" , "Sharp sound of disapproval" , "S-s-snake's s-s-sound?" .)