Is it less than seven eighths the equine? (5)

I believe the answer is:
neigh
'equine?' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'is it less than seven eighths' is the wordplay.
'NEIGH' is hidden within the letters.
But, I am uncertain how the hidden word is indicated.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for neigh that I've seen before include "Sound coming from Ben Nevis" , "Equine utterance" , "Horse's whinny" , "Negative sound of horse?" , "Whinnying sound" .)
