The clue is:
Wader, last of those being watchable to all in this lake (5)
Pattern: H _ R _ N
It looks like HERON (wader) or HURON (lake)
I went with HERON, as follows: lake = HURON, last of those = E, "watchable to all in this" = insert E = HERON
The derivation is weak, but I thought it sufficient.
The solution was HURON.
How can that be?
Other waders are: Pulver (for R), and Flamingo (for O)
"Watchable to all" could be HOT URN (ie. H URN), from "a watched pot never boils".
To me, neither derivation fits perfectly.
Which is strongest (HERON or HURON)?
Is there another derivation?