Well I know some of the others found it easy but I didn't! I did, however, find it very easy to place the words in the grid. It was obvious where "S" should go and it was plain sailing (or should I say YARE thereafter.
I still don't understand wordplay for
Wake-Robin
or
Hookworm
Also, had never heard of Tenebrae but anzwer was obvious from the cluing!
Any help with the other 2 would be much appreciated.