To split up and desert over the onset of marriage is an idealised vision (10)

'an idealised vision' is the definition.
I can't tell whether this definition defines the answer.

'to split up and desert over the onset of marriage' is the wordplay.
'split' means one lot of letters goes inside another (some letters split a word into two).
'and' says to put letters next to each other.
'desert' becomes 'sinai' (Sinai is an example).
'over' is a reversal indicator.
'the onset of' says to take the initial letters.
The first letter of 'marriage' is 'm'.
'sinai' written backwards gives 'ianis'.
'to' going inside 'up' is 'utop'.
'utop'+'ianis'+'m'='UTOPIANISM'

'is' is the link.

Can you help me to learn more?