Great frolic on the carriage (8)
I believe the answer is:
gigantic
'great' is the definition.
(I've seen this before)
'frolic on the carriage' is the wordplay.
'frolic' becomes 'antic' (I've seen this before).
'on' says to put letters next to each other.
'the carriage' becomes 'gig' (gig is a kind of carriage).
'antic' put after 'gig' is 'GIGANTIC'.
(Other definitions for gigantic that I've seen before include "Extremely large" , "Hugh,colossal" , "Fantastically great" , "'Enormous, mammoth (8)'" , "Thumping" .)