Only half grand? That's the old you! (4)
I believe the answer is:
thou
'the old you' is the definition.
(historical second person pronoun)
'only half grand?' is the wordplay.
'only half' means to take half the letters.
'grand?' becomes 'thousand' ('thousand' can be a synonym of 'grand').
'thousand' cut in half is 'THOU'.
'that's' acts as a link.
(Other definitions for thou that I've seen before include "You (many years ago)" , "once paired with art" , "Old English pronoun" , "You, in olden times" , "Old-fashioned pronoun" .)