Lengthy sojourn in the field (8)
I believe the answer is:
longstop
'the field' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'lengthy sojourn' is the wordplay.
'lengthy' becomes 'long' ('long' can be a synonym of 'lengthy').
'sojourn' becomes 'stop' (I can't justify this - if you can you should give a lot more credence to this answer).
'long'+'stop'='LONGSTOP'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for longstop that I've seen before include "one who is in deep" , "Fielding position" , "Cricketer" .)