Whopper? Large, that is (3)
I believe the answer is:
lie
'whopper?' is the definition.
(whopper can mean an outrageous lie)
'large that is' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'l'+'ie'='LIE'
(Other definitions for lie that I've seen before include "Assume recumbent posture" , "Recline - untruth" , "Falsity" , "Porky" , "Story" .)