Top of double-decker bus to crash into reservations (6)
I believe the answer is:
doubts
'reservations' is the definition.
(I've seen this before)
'top of double-decker bus to crash' is the wordplay.
'top of double decker' becomes 'd' (I can't explain this - if you can you should believe this answer much more).
'crash' indicates anagramming the letters.
'bus'+'to'='busto'
'busto' with letters rearranged gives 'oubts'.
'd'+'oubts'='DOUBTS'
'into' acts as a link.
(Other definitions for doubts that I've seen before include "Isn't certain about, has reservations" , "Uncertainties" , "Thinks something unlikely" , "Misgivings" , "Suspicions" .)