Caption giving information in form of light (6)
I believe the answer is:
legend
'caption' is the definition.
(I know that caption can be written as legend)
'information in form of light' is the wordplay.
'information' becomes 'gen' (gen can informally mean information).
'in' is an insertion indicator.
'form of light' becomes 'LED' (light-emitting diode).
'gen' placed into 'led' is 'LEGEND'.
'giving' is the link.
(Other definitions for legend that I've seen before include "explanation" , "Story about mytical beings" , "Footnote" , "Old yarn" , "Key" .)