An unequal share? Certainly! (3,4)

I believe the answer is:

not half

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

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

I can't explain the remainder of the clue.

(Other definitions for not half that I've seen before include "Too right!" , "like a bigger drink?" , "missing 50%" , "a lot" , "I couldn't agree more!" .)

Want a hint initially instead of a full solution? Install my app