Big Joe, a six-footer (5)
I believe the answer is:
giant
'big' is the definition.
(similar in meaning)
'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 "Large black and white mammal" , "Very big person" , "Titan" , "A mighty man like Goliath or Carnera" , "Huge person" .)