Mass of insects small, getting close (5)
I believe the answer is:
swarm
'mass of insects' is the definition.
(I've seen this before)
'small getting close' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'getting close' becomes 'warm' (I've seen this before).
's'+'warm'='SWARM'
(Other definitions for swarm that I've seen before include "Arrive in numbers" , "Hive" , "Congregate" , "Mass of bees" , "Large number" .)