Seeming old and wise taking time (10)
I believe the answer is:
ostensible
'seeming' is the definition.
('ostensible' can be a synonym of 'seeming')
'old and wise taking time' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'and' says to put letters next to each other.
'wise' becomes 'sensible' (I've seen this before**).
'taking' indicates putting letters inside.
'time' becomes 't'.
'sensible' enclosing 't' is 'stensible'.
'o'+'stensible'='OSTENSIBLE'
(Other definitions for ostensible that I've seen before include "Supposed" , "Seeming" , "Apparently true" , "Apparent" , "Professed" .)