Leader of crazy expedition, pure and simple (6)
I believe the answer is:
chaste
'pure and simple' is the definition.
(I've seen this before)
'leader of crazy expedition' is the wordplay.
'leader of' indicates taking the first letters.
'expedition' becomes 'haste' (I've seen this before).
The first letter of 'crazy' is 'c'.
'c'+'haste'='CHASTE'
(Other definitions for chaste that I've seen before include "Set Cha up for being pure" , "sober" , "Refraining from sex" , "virgin" , "Pure and inviolate" .)