A prom around unfinished accommodation that's shapeless (9)
I believe the answer is:
amorphous
'shapeless' is the definition.
(I know that shapeless can be written as amorphous)
'a prom around unfinished accommodation' is the wordplay.
'around' indicates an anagram.
'unfinished' means to remove the last letter.
'accommodation' becomes 'house'.
'house' with its final letter taken off is 'hous'.
'prom' with letters rearranged gives 'morp'.
'a'+'morp'+'hous'='AMORPHOUS'
'that's' acts as a link.
(Other definitions for amorphous that I've seen before include "Without distinct shape or character" , "badly organised" , "Shapeless" , "vague" , "Characterless" .)