Land in the water when one's left suspended in the middle (4)
I believe the answer is:
isle
'land in the water' is the definition.
(land surrounded by water)
'one's left suspended in the middle' is the wordplay.
'one's' becomes 'i's' (Roman numeral).
'left' becomes 'l' (common abbreviation).
'in the middle' says to take the centre.
The central letter of 'suspended' is 'e'.
'is'+'l'+'e'='ISLE'
'when' acts as a link.
(Other definitions for isle that I've seen before include "The Emerald - (Ireland)" , "Small region of land" , "This could be key" , "Geographic location" , "Dogs, for example" .)