Dead giant in play (7)
I believe the answer is:
totally
'dead' is the definition.
(synonyms)
'giant in play' is the wordplay.
'giant' becomes 'tall'.
'in' is an insertion indicator.
'play' becomes 'toy' (synonyms).
'tall' inserted into 'toy' is 'TOTALLY'.
(Other definitions for totally that I've seen before include "Absolutely" , "Dead" , "without reserve" , "Utterly" , "Altogether" .)