Being agile he put the light on first (5)
I believe the answer is:
lithe
'being agile' is the definition.
(I've seen this before)
'he put the light on first' is the wordplay.
'put the light on' becomes 'lit' (I can't justify this - if you can you should give a lot more credence to this answer).
'first' means one lot of letters go next to another.
'he' put after 'lit' is 'LITHE'.
Can you help me to learn more?
(Other definitions for lithe that I've seen before include "grace" , "able to move supply" , "Loose-limbed" , "Physically supple, agile" , "Elastic" .)