Encountered feathers around Lake, indication of disaster (8)
I believe the answer is:
meltdown
'indication of disaster' is the definition.
(a meltdown could cause a nuclear disaster)
'encountered feathers around lake' is the wordplay.
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'feathers' becomes 'down' (down is a kind of feathers).
'around' means one lot of letters goes inside another.
'lake' becomes 'l'.
'met'+'down'='metdown'
'metdown' enclosing 'l' is 'MELTDOWN'.
(Other definitions for meltdown that I've seen before include "Nuclear reactor accident" , "Serious nuclear accident" , "Disastrous event as to overheated nuclear reactor" , "Sudden disastrous failure (of nuclear reactor)" , "Disastrous collapse - nuclear accident" .)