Pig-headed group in conspiracy (4)
I believe the answer is:
plot
'conspiracy' is the definition.
(I know that conspiracy can be written as plot)
'pig-headed group' is the wordplay.
'pig-headed' becomes 'p' (start (head) of the word pig).
'group' becomes 'lot' ().
'p'+'lot'='PLOT'
'in' is the link.
(Other definitions for plot that I've seen before include "Graph; piece of ground" , "Ground - plan" , "A scheme, or a parcel of land" , "Patch of ground" , "Scheme or small piece of land" .)