Underground cell: muck one out (7)
I believe the answer is:
dungeon
'underground cell' is the definition.
(I've seen this before)
'muck one out' is the wordplay.
'muck' becomes 'dung' (synonyms).
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'one' with letters rearranged gives 'eon'.
'dung'+'eon'='DUNGEON'
(Other definitions for dungeon that I've seen before include "part of prison" , "Unpleasant cell" , "prison the result" , "old prison" , "Uncongenial accommodation" .)