Reveal speculation on a source of light (6)
I believe the answer is:
betray
'reveal' is the definition.
(I know that betray is a more specific form of the action reveal)
'speculation on a source of light' is the wordplay.
'speculation' becomes 'bet' (bet is a kind of speculation).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'a source of light' becomes 'ray'.
'bet'+'ray'='BETRAY'
(Other definitions for betray that I've seen before include "'Deceive, break trust (6)'" , "Be disloyal to like Judas" , "Deliver to an enemy by treachery" , "Cheat on" , "26 [SELL]" .)