Somehow one dug new underground cell (7)
I believe the answer is:
dungeon
'underground cell' is the definition.
(I've seen this before)
'somehow one dug new' is the wordplay.
'somehow' is an anagram indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'one'+'dug'='onedug'
'onedug' with letters rearranged gives 'dungeo'.
'dungeo'+'n'='DUNGEON'
(Other definitions for dungeon that I've seen before include "prison the result" , "Dark place of confinement" , "Underground prison-cell" , "Prison cell, often underground" , "Place of confinement underground" .)