A lot of insects needing southern heat (5)
I believe the answer is:
swarm
'a lot of insects' is the definition.
'southern heat' is the wordplay.
'southern' becomes 's' (abbreviation).
'heat' becomes 'warm' (I've seen this before).
's'+'warm'='SWARM'
'needing' acts as a link.
(Other definitions for swarm that I've seen before include "Large number" , "Congregate" , "Arrive in numbers" , "Dense group (of insects)" , "Throng of insects" .)