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 numerals).
'and' is a charade indicator (letters next to each other).
'one whales rolling' becomes 'magi' (I can't justify this - if you can you should believe this answer much more).
'c' put after 'magi' is 'MAGIC'.
'seeing' is the 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" .)