Seeming old, wise to accept retirement at last (10)
I believe the answer is:
ostensible
'seeming' is the definition.
(synonyms)
'old wise to accept retirement at last' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'wise' becomes 'sensible' (I've seen this before).
'to accept' indicates putting letters inside.
'at last' indicates one should take the final letters.
The final letter of 'retirement' is 't'.
'sensible' going around 't' is 'stensible'.
'o'+'stensible'='OSTENSIBLE'
(Other definitions for ostensible that I've seen before include "Apparent" , "Professed" , "Apparently true" , "Manifest" , "Supposed" .)