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