Heritage prison set back within Lydney's walls (6)
I believe the answer is:
legacy
'heritage' is the definition.
(synonyms)
'prison set back within lydney's walls' is the wordplay.
'prison' becomes 'cage' (cage is a kind of prison).
'set back' shows that the letters should be reversed in order.
'within' indicates putting letters inside.
'walls' says to hollow out the word (remove centre letters) (the walls are the letters on each edge).
'lydney' with its middle removed is 'ly'.
'cage' in reverse letter order is 'egac'.
'egac' inserted into 'ly' is 'LEGACY'.
(Other definitions for legacy that I've seen before include "Birthright" , "That which is handed down by a predecessor" , "estate?" , "Willed gift" , "Hand-me-down" .)