Map reveals lake in hollow (4)
I believe the answer is:
plan
'map' is the definition.
(thesaurus)
'lake in hollow' is the wordplay.
'lake' becomes 'l'.
'in' means one lot of letters goes inside another.
'hollow' becomes 'pan' (I've seen this in other clues).
'l' placed inside 'pan' is 'PLAN'.
'reveals' acts as a link.
(Other definitions for plan that I've seen before include "Design; map" , "Project" , "Road map" , "Aim" , "Scheme - plot" .)