What's more, dad's upset the boy! (5)
I believe the answer is:
added
'what's more' is the definition.
(similar in meaning)
'dad's upset the boy' is the wordplay.
'upset' indicates an anagram.
'the boy' becomes 'ed' (short for Edward).
'dad' anagrammed gives 'add'.
'add'+'ed'='ADDED'
(Other definitions for added that I've seen before include "Extra" , "Contributed" , "Said" , "Totted" , "Summed up" .)