ESP in later years keeping old in secret work (9)
I believe the answer is:
espionage
'secret work' is the definition.
(the work of a spy)
'esp in later years keeping old' is the wordplay.
'later years' becomes 'age' (old age).
'keeping' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'in'+'age'='inage'
'inage' placed around 'o' is 'ionage'.
'esp'+'ionage'='ESPIONAGE'
'in' is the link.
(Other definitions for espionage that I've seen before include "secret work" , "hush-hush work" , "intelligence" , "undercover work" , "Practice of spying" .)