Take a seat during procession without duke, a hanger-on (8)
I believe the answer is:
parasite
'a hanger-on' is the definition.
(thesaurus)
'take a seat during procession without duke' is the wordplay.
'take a seat' becomes 'sit' (I've seen this before).
'during' means one lot of letters goes inside another.
'procession' becomes 'parade' (parade is a kind of procession).
'without' suggests deleting specific letters.
'duke' becomes 'd'.
'sit' going within 'parade' is 'paradsite'.
'paradsite' with 'd' taken away is 'PARASITE'.
(Other definitions for parasite that I've seen before include "unwanted guest" , "Creature that lives off others" , "sponge" , "Animal or plant that lives off another" , "Organism living off another" .)