Ultimately colossal, mega, bumper, big, huge (5)
I believe the answer is:
large
'huge' is the definition.
(similar in meaning)
'ultimately colossal mega bumper big' is the wordplay.
'ultimately' suggests the final letters.
'big' becomes 'ge' (I can't explain this - if you can you should give a lot more credence to this answer).
The last letters of 'colossal mega bumper' are 'lar'.
'lar'+'ge'='LARGE'
Can you help me to learn more?
(Other definitions for large that I've seen before include "Huge" , "Comprehensive" , "jumbo" , "Bulky" , "broad" .)