Some insects are small and affectionate (5)
I believe the answer is:
swarm
'some insects' is the definition.
(I have seen 'many insects' mean 'swarm' so perhaps 'insects' could also mean 'swarm')
'small and affectionate' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'and' means one lot of letters go next to another.
'affectionate' becomes 'warm' (synonyms).
's'+'warm'='SWARM'
'are' is the link.
(Other definitions for swarm that I've seen before include "Body of bees" , "Large group of bees" , "Mob" , "Arrive in numbers" , "Dense group (of insects)" .)