Heathen almost incarcerates old duke in temple (6)
I believe the answer is:
pagoda
'temple' is the definition.
(pagoda is a kind of temple)
'heathen almost incarcerates old duke' is the wordplay.
'heathen' becomes 'pagan' (both can mean an irreligious person).
'almost' means to remove the last letter (almost the whole word is kept).
'incarcerates' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'duke' becomes 'd' (abbreviation).
'pagan' with its final letter removed is 'paga'.
'o'+'d'='od'
'paga' going around 'od' is 'PAGODA'.
'in' acts as a link.
(Other definitions for pagoda that I've seen before include "shrine" , "Pyramidal Asian temple" , "Pyramid-shaped Asian temple" , "Eastern temple, usually a tower" , "A buddhist temple built in tiers" .)