Automatic machine — covers are on top (4)
I believe the answer is:
acme
'top' is the definition.
(synonyms)
'automatic machine covers are on' is the wordplay.
'automatic machine' becomes 'cm' (I am not sure about this - if you are sure you should believe this answer much more).
'covers' says to hollow out the word (remove centre letters) (I've seen 'covers of' mean this).
'on' is an insertion indicator (as in clothing 'on' a person).
'are' with its centre taken out is 'ae'.
'cm' inserted inside 'ae' is 'ACME'.
Can you help me to learn more?
(Other definitions for acme that I've seen before include "Aim of mountaineers" , "crisis in history" , "Peak of excellence" , "Summit - zenith" , "Summit - maximum" .)