Grub that's eventually become perfect! (5)
I believe the answer is:
imago
'grub' is the definition.
The definition and answer can be both animals as well as being singular nouns.
Maybe there's an association between them I don't understand?
'eventually become perfect' is the wordplay.
I cannot really understand how this works, but
'become' could be 'go' (eg to go rotten is to become rotten) and 'go' is located in the answer.
The remaining letters 'ima' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'that's' acts as a 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" .)