Joe, a six-footer, is very big (5)
I believe the answer is:
giant
'is very big' is the definition.
(I've seen this before)
'joe a six-footer' is the wordplay.
'joe' becomes 'GI' (GI Joe).
'a six footer' becomes 'ant' (an ant has six feet).
'gi'+'ant'='GIANT'
(Other definitions for giant that I've seen before include "Creature of exceptional size" , "Titan" , "Huge; James Dean film" , "Extremely large (person)" , "Designating big" .)