25 across
Youthful
follower
of
steam
engines
(6)
Youthful follower of steam engines (6)
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The whole clue is the definition (& lit or double definition).
How the clue works
The definition is Youthful — tender meaning young and delicate. Follower of steam engines is your second definition, since a tender is also the car coupled behind a locomotive to carry its fuel and water (a lovely double definition, this one).