5 down
X÷e
=
13-19
(4)
X÷e = 13-19 (4)
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The definition is at the end of the clue.
How the clue works
The definition is 13-19, the range of ages in which one is know as "teen". For the wordplay, "X" is the Roman numeral for TEN, and "÷" (divided by) is the container indicator telling us to split TEN around an "e", giving T-E-EN.