Son has the best yell (6)
I believe the answer is:
scream
'yell' is the definition.
('scream' can be a synonym of 'yell')
'son has the best' is the wordplay.
'son' becomes 's'.
'has' says to put letters next to each other.
'the best' becomes 'cream' (I've seen this before).
's'+'cream'='SCREAM'
(Other definitions for scream that I've seen before include "Loud piercing cry" , "A shrill yell as from fright" , "Utter a sudden loud cry" , "Extremely funny entertainer" , "Howl, shriek" .)