A thousand donkeys? That's a lot (6)
I believe the answer is:
masses
'a lot' is the definition.
(I've seen this before)
'a thousand donkeys?' is the wordplay.
'a thousand' becomes 'm' (Roman numeral for a thousand).
'donkeys?' becomes 'asses' (an ass is a donkey).
'm'+'asses'='MASSES'
'that's' acts as a link.
(Other definitions for masses that I've seen before include "A lot" , "Crowds of common people" , "Large numbers or amounts" , "Loads" , "Great numbers, at religious ceremonies?" .)