Physicist initially well-informed about light particle (6)
I believe the answer is:
photon
'light particle' is the definition.
(I've seen this in another clue)
'physicist initially well-informed about' is the wordplay.
'initially' indicates taking the first letters.
'well informed' becomes 'hot' (being hot on a topic is being informed about it).
'about' becomes 'on' (as in a book 'on' a certain topic).
The first letter of 'physicist' is 'p'.
'p'+'hot'+'on'='PHOTON'
(Other definitions for photon that I've seen before include "Quantum particle of light" , "A little light" , "Tiny bit of light" , "Quantum of electromagnetic radiation" .)