Crossword Genius

After halfway, swim back, butterfly? (5)

Ross

I believe the answer is:

imago

I'm a little stuck... Click here to teach me more about this clue!

'butterfly?' is the definition.
(I've seen this before)

'after halfway swim back' is the wordplay.
'after halfway swim' becomes 'im' (I can't explain this - if you can you should believe this answer much more).
'back' becomes 'ago' ('years back' means 'years ago').
'im'+'ago'='IMAGO'

Can you help me to learn more?

(Other definitions for imago that I've seen before include "Fully developed insect" , "That's as far as a butterfly can go" , "Last stage of adult insect" , "Insect in its adult form" , "Sexually mature adult insect" .)

I've seen this clue in The Independent.
Want a hint initially instead of a full solution? Install my app