Both of us said, 'this is fun! (4)
I believe the answer is:
whee
''this is fun' is the definition.
(wheeeeeeeeeeeeeeee)
'both of us said' is the wordplay.
'both of us' becomes 'we' (you and I).
'said' shows a homophone (sound like).
'we' sounds like 'WHEE'.
(Other definitions for whee that I've seen before include "Exclamation of excitement when going fast" , "I'm going downhill, fast!" , "Expression of delight" , "What fun" , "Cry from someone going downhill fast?" .)