Debt-collector gone mad in underground cell (7)
I believe the answer is:
dungeon
'underground cell' is the definition.
(I've seen this before)
'debt-collector gone mad in' is the wordplay.
'debt collector' becomes 'dun' (I've seen this in other clues).
'mad' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'gone' anagrammed gives 'ngeo'.
'dun' placed around 'ngeo' is 'DUNGEON'.
(Other definitions for dungeon that I've seen before include "old prison" , "prison the result" , "Prison cell, often underground" , "Unpleasant cell" , "Uncongenial accommodation" .)