Taxi with a large group of conspirators (5)
I believe the answer is:
cabal
'group of conspirators' is the definition.
(I've seen this in another clue)
'taxi with a large' is the wordplay.
'taxi' becomes 'cab' (synonyms).
'with' is a charade indicator (letters next to each other).
'large' becomes 'L' (eg in clothes sizes).
'cab'+'a'+'l'='CABAL'
(Other definitions for cabal that I've seen before include "Secret plot; clique" , "Camp" , "Intrigue, clique" , "Secret political group" , "Secret group of plotters" .)