A leading poet’s with expedition (5)
I believe the answer is:
amain
'poet's with expedition' is the definition.
(poetic term meaning quickly or expeditiously)
'a leading' is the wordplay.
'leading' becomes 'main' (I've seen this in another clue).
'a'+'main'='AMAIN'
(Other definitions for amain that I've seen before include "at full speed" , "powerfully poetic" , "at great speed" , "showing great strength in poetry" , "in great haste" .)