Very old boxer turning up — there you are! (5)
I believe the answer is:
voila
'there you are' is the definition.
(I've seen this before)
'very old boxer turning up' is the wordplay.
'very' becomes 'v' (abbreviation).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'boxer' becomes 'Ali' (Muhammad Ali).
'turning up' says the letters should be written backwards.
'ali' in reverse letter order is 'ila'.
'v'+'o'+'ila'='VOILA'
(Other definitions for voila that I've seen before include "Announcement from French producer" , "French interjection - there you are!" , "See there, behold - in France" , "The French see" , "See there, howzat - in France" .)