Multitude first seen getting closer (5)
I believe the answer is:
swarm
'multitude' is the definition.
(thesaurus)
'first seen getting closer' is the wordplay.
'first seen' becomes 's' (1st letter of 'seen').
'getting closer' becomes 'warm' (I've seen this in another clue).
's'+'warm'='SWARM'
(Other definitions for swarm that I've seen before include "Body of bees" , "Group of flies" , "Climb; cluster of bees" , "Flight of bees" , "Dense group (of insects)" .)