Cunning, extremely deliberate, firm style of design (3,4)
I believe the answer is:
art deco
'style of design' is the definition.
(I've seen this before)
'cunning extremely deliberate firm' is the wordplay.
'cunning' becomes 'art' (both can mean trickery).
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'firm' becomes 'co' (a firm is a company).
'deliberate' with its centre removed is 'de'.
'art'+'de'+'co'='ART DECO'
(Other definitions for art deco that I've seen before include "modern style" , "Geometric style of ornamentation" , "Visual design style - redcoat (anag)" , "Style characterised by bold geometric shapes" , "Nineteen twenty's and thirties style" .)