Marvellous seeing a hundred and one whales rolling (5)
I believe the answer is:
magic
'marvellous' is the definition.
(I've seen this before)
'a hundred and one whales rolling' is the wordplay.
'a hundred' becomes 'C' (Roman numeral).
'and' means one lot of letters go next to another.
'one whales rolling' becomes 'magi' (I can't explain this - if you can you should believe this answer much more).
'c' after 'magi' is 'MAGIC'.
'seeing' acts as a link.
Can you help me to learn more?
(Other definitions for magic that I've seen before include "Great" , "Tricks" , "Supernatural power" , "Conjuring trick, legerdemain" , "Wonderful" .)