Highlight the white text inside the block for hints ranging from vague to specific...
Start by solving the clues using only the letters found in HYDRAE.
The starting letters for each clue are:
Titaness mother of the Olympian deities: R
Daughter and anagram of that titaness: H
One of the 20 Odysseus spent wandering: Y
Measurement of threee feet: Y
Tough and resilient: H
Feeling of eerie discontent: D
Parent, familiarly: D
Paid attention to: H
Bleak and ominous: D
One with a flock: H
Time of greatest accomplishment: H
One called "Firehair," perhaps: R
Calculated a new sum: R
Attached with glue: A
What there might not be in the house when sad songs play: A
Transfer the letters ABOVE the numbers in the clue answers to the grid. Double-check this 3 times, because it is super-easy to switch to adding the letters below the numbers to the grid instead.
The directions are a little ambiguous, so to clarify: all of the letters in one instance of the word "HYDRA" are part of one head. No letter from one head is orthogonally adjacent to any other letter from another head. They are diagonally adjacent, however. Also, when it says "Each of the squares in the grid will contain a letter when you're done" what they are saying is fill in a letter E whenever a cell cannot contain a letter from a head due to a head "looking" in that direction. Personally, I find it easier to fill a cell with a dot so that the letters for the word HYDRA pop out cleaner.
The middle two rows of the grid are as follows:
The next section contains the answer. Do not reveal unless you want to be spoiled