Crowd is small and affectionate (5)
I believe the answer is:
swarm
'crowd' is the definition.
(I know that swarm is a type of crowd)
'small and affectionate' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'and' is a charade indicator (letters next to each other).
'affectionate' becomes 'warm' (synonyms).
's'+'warm'='SWARM'
'is' acts as a link.
(Other definitions for swarm that I've seen before include "Large group of flying insects" , "Large number" , "Crowd of e.g. bees" , "Dense group (of insects)" , "Mass of bees" .)