Old girl, one getting on, missing out (8)
I believe the answer is:
omission
'missing out' is the definition.
'old girl one getting on' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'girl' becomes 'miss' (I've seen this before**).
'one' becomes 'i' (Roman numeral).
'getting' is a charade indicator (letters next to each other).
'o'+'miss'+'i'+'on'='OMISSION'
(Other definitions for omission that I've seen before include "type of sin" , "A thing left out" , "lapse" , "Something left out or forgotten" , "Mistake resulting from neglect" .)