Man perhaps, shortly saying 'I will' (4)
I believe the answer is:
isle
'man perhaps' is the definition.
(I've seen this before)
'shortly saying i will' is the wordplay.
'shortly' means to remove the first letter.
'saying' indicates a 'sounds like' (homophone) clue.
'i will' becomes 'ill' (I've seen this before).
'ill' is a homophone of 'aisle'.
'aisle' with its initial letter taken away is 'ISLE'.
(Other definitions for isle that I've seen before include "being impossible without water" , "eg Man, Wight" , "Capri for one" , "Land amid water" , "Small region of land" .)