Conceited one among the leaders (4)
I believe the answer is:
vain
'conceited' is the definition.
(I know that conceited can be written as vain)
'one among the leaders' is the wordplay.
'one' becomes 'i' (Roman numeral).
'among' indicates putting letters inside (inserted letters are placed among others).
'the leaders' becomes 'van' (I've seen this in another clue).
'i' placed into 'van' is 'VAIN'.
(Other definitions for vain that I've seen before include "It's no good" , "Conceited or unsuccessful" , "Stuck-up? It's hopeless" , "being a big-head" , "Useless; self-regarding" .)