Strongest eight lost in fog (9)
I believe the answer is:
mightiest
'strongest' is the definition.
(I've seen this before)
'eight lost in fog' is the wordplay.
'lost' indicates an anagram.
'in' indicates putting letters inside.
'fog' becomes 'mist' (mist is a kind of fog).
'eight' is an anagram of 'ghtie'.
'ghtie' put into 'mist' is 'MIGHTIEST'.
(Other definitions for mightiest that I've seen before include "Strongest" , "Most powerful" .)