Northbound David in an American state (6)
I believe the answer is:
nevada
'american state' is the definition.
(I know that Nevada is a U.S. state)
'northbound david in an' is the wordplay.
'northbound' is a reversal indicator.
'david' becomes 'dave' (Dave is short for David).
'in' means one lot of letters goes inside another.
'dave' put into 'an' is 'adaven'.
'adaven' backwards is 'NEVADA'.
(Other definitions for nevada that I've seen before include "See 14" , "US state, capital Carson City" , "part of US" , "State of western USA" , "State including Las Vegas" .)