Battle site is somewhat muddy, presumably (5)
I believe the answer is:
Here is my best explanation:
'battle site' is the definition.
(I've seen this before)
'somewhat muddy presumably' is the wordplay.
'somewhat' indicates the answer is hidden within the clue.
'YPRES' is hidden within 'muddy presumably'.
'is' is the link.
(Other definitions for ypres that I've seen before include "WWI battlefield", "Belgian World War I battle site", "World War I battleground", "Scene of action", "Belgian town", "Belgian World War One battle site", "World War I battlefield", "Battlefield", "WW One battle site" and "Battle".)