Sets firm with unusual angles (8)
I believe the answer is:
congeals
'sets' is the definition.
('congeal' can be a synonym of 'set')
'firm with unusual angles' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'with' means one lot of letters go next to another.
'unusual' indicates anagramming the letters.
'angles' with letters rearranged gives 'ngeals'.
'co'+'ngeals'='CONGEALS'
(Other definitions for congeals that I've seen before include "Becomes gelatinous" , "Coagulates" .)