Horse and dope found around old prison cell (7)
I believe the answer is:
dungeon
'prison cell' is the definition.
(I've seen this before)
'horse and dope found around old' is the wordplay.
'horse' becomes 'dun' (I've seen this before).
'and' means one lot of letters go next to another.
'dope' becomes 'gen' (both can mean information).
'found around' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'gen' enclosing 'o' is 'geon'.
'dun'+'geon'='DUNGEON'
(Other definitions for dungeon that I've seen before include "Unpleasant cell" , "Uncongenial accommodation" , "Underground prison-cell" , "Keep" , "Dark place of confinement" .)