Horribly large (5)
'horribly large' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for gross that I've seen before include "Earn" , "Rank" , "large total before deductions" , "Make altogether" , "exclamation of disgust" .)
(Other definitions for monstrous that I've seen before include "Shocking; huge" , "Terrible" , "really good!" , "Quite outrageous" , "Extraordinarily large" .)