Fly-by-night getting married in the morning -- first whopper! (7)
I believe the answer is:
mammoth
'fly-by-night' is the definition.
Both the answer and definition are adjectives. Maybe there's a link between them I don't understand?
'getting married in the morning first whopper' is the wordplay.
'getting' is an insertion indicator.
'married' becomes 'm' (abbreviation, e.g. in genealogy).
'in the' means one lot of letters goes inside another.
'morning' becomes 'am' (as in mo:00 AM).
'first whopper' becomes 'moth' (I can't explain this - if you can you should believe this answer much more).
'm' placed within 'am' is 'amm'.
'amm' inserted within 'moth' is 'MAMMOTH'.
Can you help me to learn more?
(Other definitions for mammoth that I've seen before include "Enormous" , "Large extinct elephant" , "Large 15 animal" , "Extinct hairy elephant" , "Large, woolly ice-age beast" .)