Swarm that’s somewhat reduced in heat (6)
I believe the answer is:
warmth
'heat' is the definition.
(I know that heat can be written as warmth)
'swarm that's somewhat reduced' is the wordplay.
'somewhat reduced' indicates a hidden word.
'WARMTH' is hidden in the letters of 'swarm that'.
'in' acts as a link.
(Other definitions for warmth that I've seen before include "Mild heat" , "Comfort" , "tenderness" , "Ardour" , "Feeling" .)