If an edit has improved the question but left more improvement still to be done, then by all means go ahead and do that improvement - just as you would have done if you'd come across the question after the edit had been approved and noticed there was still more improvement to be done.
The system even goes out of its way to make it easier for you to do this:
There are not just buttons for Approve and Reject (the simplest possibilities), but also Improve Edit (which automatically approves the suggested edit, giving that user their +2 rep, and then puts you into the edit box so you can re-edit the edited version of the question) and Reject and Edit (which automatically rejects the suggested edit and puts you into the edit box so you can edit the original version of the question).
For more details on all facets of the suggesting and reviewing edits process, see this post on main meta.