Very variable, casual apology (3)

'casual apology' is the definition.
(short for sorry)

'very variable' is the wordplay.
I cannot quite understand how this works, but
'very' could be 'so' (both can mean 'extremely') and 'so' is found in the answer.
'variable' could be 'z' (algebra) and 'z' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...