Appropriate, your being old and secretive (8)
I believe the answer is:
stealthy
'secretive' is the definition.
(I've seen this before)
'appropriate your' is the wordplay.
'appropriate' becomes 'steal' (appropriate can mean to steal).
'your' becomes 'thy' ('thy' historically meant 'your').
'steal'+'thy'='STEALTHY'
'being old and' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for stealthy that I've seen before include "Unobtrusive" , "'Sneaky, clandestine (8)'" , "'Furtive, surreptitious (8)'" , "Covert - clandestine" , "Moving surreptitiously" .)