Importuner ultimately serving a long time in prison cell (7)
I believe the answer is:
dungeon
'prison cell' is the definition.
(I know that Dungeon is a type of prison cell)
'importuner ultimately serving a long time' is the wordplay.
'importuner' becomes 'dun' (dun can mean someone who importunes).
'ultimately' says to take the final letters.
'a long time' becomes 'eon' (eon is a kind of long time).
The final letter of 'serving' is 'g'.
'dun'+'g'+'eon'='DUNGEON'
'in' acts as a link.
(Other definitions for dungeon that I've seen before include "Dark underground cell for prisoners" , "Underground prison-cell" , "Prison cell, often underground" , "Eg, the Black Hole of Calcutta" , "Unpleasant cell" .)