Old political group ultimately deficient in style (2,3)
I believe the answer is:
op art
'style' is the definition.
(I have seen 'Style of painting ' mean 'op art' so perhaps 'style' could also mean 'op art')
'old political group ultimately deficient' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'political group' becomes 'party' (I've seen this before).
'ultimately deficient' means to remove the last letter (the word lacks its last or ultimate letter).
'party' with its final letter taken away is 'part'.
'o'+'part'='OP ART'
'in' acts as a link.
(Other definitions for op art that I've seen before include "Illusory graphic style" , "that may give the illusion of movement" , "Illusory works" , "Illusory graphic work" , "Illusory design style" .)