There is no theme and no letter count 4A + 600 + EN, rearranged...
When I posted this before an answer of Ascend was suggested which relies on the 4A part of the clue resulting in a’s in the answer. I’m very sceptical of that because why would the clue specify 4A when it could equally have said 3A or 5A etc which would lead to the same answer.
So I’ve re-posted the question to see if anyone has any other ideas?