Stallion gets big (4)
I believe the answer is:
tall
'big' is the definition.
(thesaurus)
'stallion gets' is the wordplay.
'gets' says the answer is hidden in the clue.
'TALL' is hidden within 'stallion'.
(Other definitions for tall that I've seen before include "Towering" , "Lofty; unlikely" , "Above average height" , "Of great stature" , "that's hard to swallow" .)