Second best son cries (7)
I believe the answer is:
screams
'cries' is the definition.
(scream is a kind of cry)
'second best son' is the wordplay.
'second' becomes 's' (common abbreviation).
'best' becomes 'cream' (I've seen this before).
'son' becomes 's'.
's'+'cream'+'s'='SCREAMS'
(Other definitions for screams that I've seen before include "Shouts" , "Shrill cries" , "Yells" .)