There are Ten lines down
These are all six letter words that link with each other
the word on the left links with the word on the right
Each link word has one shaded square in it.
When all ten linkword lines are filled in, the shaded letters make up the clue
that is the main answer to the clue.
The clue in this case is " Living Fossil Fish" ( 10)
answer "COELACANTH" one letter of that word in each line
I hope I have explained ok