Boy, I understand that! (5)
I believe the answer is:
roger
I believe this is a double definition.
'boy' is the first definition.
(I've seen this before)
'i understand that' is the second definition.
(roger that)
(Other definitions for roger that I've seen before include "Boy - received and understood" , "... guy" , "Man's name" , "One like de Coverley" , "Expression of agreement" .)