Rollercoaster beginning to buckle, one gripped tight (3,6)
I believe the answer is:
big dipper
'rollercoaster' is the definition.
(I've seen this before)
'beginning to buckle one gripped tight' is the wordplay.
'beginning to' says to take the initial letters.
'one' becomes 'I' (Roman numeral).
'tight' is an anagram indicator (tight can mean drunk).
The initial letter of 'buckle' is 'b'.
'gripped' is an anagram of 'gdipper'.
'b'+'i'+'gdipper'='BIG DIPPER'
(Other definitions for big dipper that I've seen before include "Amusement park attraction" , "Stars in America" , "North American 24 across" , "Familiar US stars" , "Large bird" .)