Leader should abandon modest expedition (5)
I believe the answer is:
haste
'expedition' is the definition.
(expedition is expediting, doing something quickly)
'leader should abandon modest' is the wordplay.
'leader should abandon' means to remove the first letter.
'modest' becomes 'chaste' (I've seen this before).
'chaste' with its first letter taken off is 'HASTE'.
(Other definitions for haste that I've seen before include "Undue speed" , "Inconsiderate speed" , "Speediness or rush" , "Expeditiousness" , "More of this can slow you up!" .)