Oddly, liar knew it should be indicated on map (4)
I believe the answer is:
lake
'it should be indicated on map' is the definition.
The definition suggests a singular noun which matches the answer.
'oddly liar knew' is the wordplay.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
'liar'+'knew'='liarknew'
The alternate letters of 'liarknew' are 'LAKE'.
Can you help me to learn more?
(Other definitions for lake that I've seen before include "Water surrounded by land" , "Superior maybe < that's red" , "Superior, for instance" , "Reservoir" , "Mere - organic pigment" .)