Ask for one new start (5)
I believe the answer is:
begin
'start' is the definition.
('begin' can be a synonym of 'start')
'ask for one new' is the wordplay.
'ask for' becomes 'beg' (begging is a kind of asking for).
'one' becomes 'i' (Roman numeral**).
'new' becomes 'n'.
'beg'+'i'+'n'='BEGIN'
(Other definitions for begin that I've seen before include "Launch" , "Institute" , "Commence" , "Make a start" , "Clandestine, furtive" .)