Boxer initially upset over Sky broadcast (4)

'boxer' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'initially upset over sky broadcast' is the wordplay.
'initially' says to take the initial letters.
'over' says to put letters next to each other (in a down clue, some letters go over others).
'broadcast' is an anagram indicator (letters scattered or cast around).
The first letter of 'upset' is 'u'.
'sky' with letters rearranged gives 'syk'.
'u'+'syk'='USYK'

Can you help me to learn more?