Capital may be arranged retrospectively (8)
I believe the answer is:
canberra
'capital' is the definition.
(capital of Australia)
'may be arranged retrospectively' is the wordplay.
'may' becomes 'can' (synonyms).
'arranged' becomes 'arr' (abbreviation).
'retrospectively' says the letters should be written in reverse.
'arr' in reverse letter order is 'rra'.
'can'+'be'+'rra'='CANBERRA'
(Other definitions for canberra that I've seen before include "Australian capital - old ship" , "Capital - bomber - liner" , "Nab racer, oddly, in Australia" , "Ban carer from this city" , "Capital of Australia" .)