Many insects are small and affectionate (5)
I believe the answer is:
swarm
'many insects' is the definition.
(eg a swarm of bees)
'small and affectionate' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'and' says to put letters next to each other.
'affectionate' becomes 'warm' (synonyms).
's'+'warm'='SWARM'
'are' acts as a link.
(Other definitions for swarm that I've seen before include "Body of bees" , "Throng of insects" , "Large group of flying insects" , "Mass of flying insects" , "Large crowd" .)