After halfway, swim back, butterfly? (5)
I believe the answer is:
imago
'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" .)