Seeming old and sober — about time (10)
I believe the answer is:
ostensible
'seeming' is the definition.
('ostensible' can be a synonym of 'seeming')
'old and sober about time' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English**).
'and' is a charade indicator (letters next to each other).
'sober' becomes 'sensible'.
'about' indicates putting letters inside.
'time' becomes 't'.
'sensible' placed around 't' is 'stensible'.
'o'+'stensible'='OSTENSIBLE'
(Other definitions for ostensible that I've seen before include "Manifest" , "Seeming" , "Apparent" , "Supposed" , "Professed" .)