Ban chemical in salt initially exterminating crustacean (8)
I believe the answer is:
barnacle
'crustacean' is the definition.
(I know that Barnacle is a type of crustacean)
'ban chemical in salt initially exterminating' is the wordplay.
'ban' becomes 'bar' (I've seen this in other clues).
'chemical in salt' becomes 'Na Cl' (chemical formula for sodium chloride).
'initially' says to take the initial letters.
The initial letter of 'exterminating' is 'e'.
'bar'+'nacl'+'e'='BARNACLE'
(Other definitions for barnacle that I've seen before include "Crustacean attached to rocks" , "On the coast it's hard to remove" , "Hull resident (one who clings on)?" , "Crustacean that attaches to submerged surfaces" , "Clinging crustacean" .)