Scene finally hidden by coastal fog certainly not the most pleasant (8)
I believe the answer is:
seamiest
'certainly not the most pleasant' is the definition.
(seamy means sordid or unpleasant)
'scene finally hidden by coastal fog' is the wordplay.
'finally' says to take the final letters.
'hidden by' means one lot of letters goes inside another (inserted letters are hidden inside).
'coastal fog' becomes 'sea mist' (mist by the sea).
The last letter of 'scene' is 'e'.
'e' placed within 'seamist' is 'SEAMIEST'.
(Other definitions for seamiest that I've seen before include "extremely unpleasant" , "Most disreputable" .)