Pretentious political group lacks leadership (4)
I believe the answer is:
arty
'pretentious' is the definition.
(similar in meaning)
'political group lacks leadership' is the wordplay.
'political group' becomes 'party' (a political party).
'lacks leadership' means to remove the first letter (I've seen 'lacking leadership' mean this).
'party' with its first letter taken off is 'ARTY'.
(Other definitions for arty that I've seen before include "Keen on paintings etc" , "Pretentiously cultural" , "Having pretensions about paintings, music, etc" , "Affectedly highbrow" , "Affectedly unconventional" .)