‘Stallions’ has three vowels — that’s academic (8)
I believe the answer is:
studious
'academic' is the definition.
(I've seen this before)
'stallions has three vowels' is the wordplay.
'three vowels' means three replacements for 'vowel'.
'stallions' becomes 'studs' (stud is a kind of stallion).
'has' is an insertion indicator.
'vowel' becomes 'i'.
'vowel' becomes 'o'.
'vowel' becomes 'u'.
'i'+'o'+'u'='iou'
'studs' going around 'iou' is 'STUDIOUS'.
'that's' acts as a link.
(Other definitions for studious that I've seen before include "Like a good schoolboy" , "reading a lot" , "Diligent, assiduous" , "Deliberate and painstaking as in reading" , "Bookish, sedulous" .)