Confine knight in the City a long time (6)
I believe the answer is:
encage
'confine' is the definition.
(I've seen this in another clue)
'knight in the city a long time' is the wordplay.
'knight' becomes 'n' (chess abbreviation).
'in the' is an insertion indicator.
'city' becomes 'EC' (the City of London postcode).
'a long time' becomes 'age' (an age is a period of time).
'n' put within 'ec' is 'enc'.
'enc'+'age'='ENCAGE'
(Other definitions for encage that I've seen before include "Bang up" , "Put behind bars" , "Pen" .)