Crossword Genius

When fired up he's agile (5)

Ross

I believe the answer is:

lithe

I'm a little stuck... Click here to teach me more about this clue!

'agile' is the definition.
(I've seen this in another clue)

'when fired up he's' is the wordplay.
'when fired' becomes 'til' (I can't explain this - if you can you should give a lot more credence to this answer).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'til' in reverse letter order is 'lit'.
'lit'+'he'='LITHE'

Can you help me to learn more?

(Other definitions for lithe that I've seen before include "Slim and supple" , "Lissom, supple" , "showing great athletic ability" , "Of body, flexible and supple" , "Supple and limber" .)

I've seen this clue in the Sydney Morning Herald.
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