Source of light in the middle of equinox month (3)
I believe the answer is:
arc
'source of light' is the definition.
'arc' can be an answer for 'light' (arc light is a kind of light). I'm not sure about the 'source of' bit.
'middle of equinox month' is the wordplay.
'middle of' indicates the central letters.
'equinox month' becomes 'march' (I've seen this in another clue).
The middle letters of 'march' are 'ARC'.
'in the' acts as a link.
Can you help me to learn more?
(Other definitions for arc that I've seen before include "Curved path" , "Luminous electrical discharge" , "circle? Not entirely" , "Segment of circle" , "Curved part of circle" .)