When fired up he's agile (5)
I believe the answer is:
lithe
'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" .)