Have doubts about American parking in camp (7)
I believe the answer is:
suspect
'have doubts about' is the definition.
(I've seen this before)
'american parking in camp' is the wordplay.
'american' becomes 'us'.
'parking' becomes 'p'.
'in' means one lot of letters goes inside another.
'camp' becomes 'sect' (sect is a kind of camp).
'us'+'p'='usp'
'usp' inserted within 'sect' is 'SUSPECT'.
(Other definitions for suspect that I've seen before include "Doubt - man held by police" , "Believe (someone) guilty" , "Believe to be guilty, but without proof" , "Imagine to be true" , "One believed responsible" .)