Battle site is somewhat muddy, presumably (5)
I believe the answer is:
ypres
'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 "WW One battle site" , "Scene of action" , "WW1 battle town" , "Belgian World War One battle site" , "scene of battle" .)