The masses going south get moderately hot sun (6)
I believe the answer is:
swarms
'the masses' is the definition.
'going south get moderately hot sun' is the wordplay.
'going south' becomes 's' (abbreviation. I am not sure about the 'going' bit.).
'get' means one lot of letters go next to another.
'moderately hot' becomes 'warm' (I've seen this before).
'sun' becomes 'S' (abbreviation).
's'+'warm'+'s'='SWARMS'
Can you help me to learn more?
(Other definitions for swarms that I've seen before include "'Moving crowds, of bees say (6)'" , "Large groups of flying bees" , "Dense groups of flying insects" .)