Gathering in multitudes to make music; inside is thermally pleasant (8)
I believe the answer is:
swarming
'gathering' is the definition.
(I've seen this before)
'multitudes to make music inside is thermally pleasant' is the wordplay.
I cannot really understand how this works, but
'music' could be 'g' and 'g' is located in the answer.
'inside' could be 'in' and 'in' is located in the answer.
The remaining letters 'swarm' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'in' acts as a link.
Can you help me to learn more?
(Other definitions for swarming that I've seen before include "moving in large numbers" , "Moving as a cloud of insects" , "buzzing about" , "in crowds" , "Gathering" .)