Begin or open with tune that's arranged? That's about right (5,4)

'begin' is the definition.
(entering upon is a kind of begining)

'open with tune that's arranged? that's about right' is the wordplay.
'with' means one lot of letters go next to another.
'that's arranged?' indicates anagramming the letters (letters rearranged).
'that's about' is an insertion indicator.
'right' becomes 'r' (abbreviation).
'open' put after 'tune' is 'tuneopen'.
'tuneopen' is an anagram of 'enteupon'.
'enteupon' going around 'r' is 'ENTER UPON'.

'or' acts as a link.