Grub that's eventually become perfect! (5)
I believe the answer is:
imago
'grub' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'eventually become perfect' is the wordplay.
I cannot really see how this works, but
'become' could be 'go' (eg to go rotten is to become rotten) and 'go' is located in the answer.
This explanation may well be incorrect...
'that's' is the link.
Can you help me to learn more?
(Other definitions for imago that I've seen before include "being immature?" , "Insect's adult phase" , "Beetle, for example" , "picture of important person" , "Maybe comma" .)