Lake boundary's shelf of rocks (5)
I believe the answer is:
ledge
'shelf of rocks' is the definition.
'ledge' can be an answer for 'rocks' (thesaurus). I'm not certain of the 'shelf of' bit.
'lake boundary's' is the wordplay.
'lake' becomes 'L' (geographical abbreviation).
'boundary' becomes 'edge' (edge is a kind of boundary).
'l'+'edge'='LEDGE'
Can you help me to learn more?
(Other definitions for ledge that I've seen before include "Sill" , "Shelf-life projection" , "Narrow projecting shelf, on cliff or building" , "Shelf-like projection" , "Narrow shelf, often attached to window" .)