First of all everybody quite understands if niece eats like a horse (6)
I believe the answer is:
equine
'like a horse' is the definition.
(relating to equine animals)
'first of all everybody quite understands if niece eats' is the wordplay.
'first of all' suggests taking the first letters.
The initial letters of 'everybody quite understands if niece eats' is 'EQUINE'.
(Other definitions for equine that I've seen before include "Like horses" , "Horse or donkey, perhaps" , "Horsey" , "like knight?" , "Horsy" .)