A topping editor, but very unpopular! (5)
I believe the answer is:
hated
'unpopular' is the definition.
(I've seen this before)
'a topping editor but very' is the wordplay.
'a topping' means one lot of letters go next to another.
'editor' becomes 'ed'.
'but very' becomes 'hat' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ed' after 'hat' is 'HATED'.
Can you help me to learn more?
(Other definitions for hated that I've seen before include "viewed with hostility" , "Despised" , "gave stinking review to?" , "Heartily disliked" , "Abominated" .)