Enemy crossing lake ice (4)
I believe the answer is:
floe
'ice' is the definition.
(thesaurus)
'enemy crossing lake' is the wordplay.
'enemy' becomes 'foe' (I've seen this before).
'crossing' indicates putting letters inside.
'lake' becomes 'l'.
'foe' enclosing 'l' is 'FLOE'.
(Other definitions for floe that I've seen before include "Sheet of ice floating in the sea" , "Comparatively small piece of floating ice" , "Broken piece of pack-ice" , "A large lump of floating ice" , "A field of floating ice" .)