Shepherd firstly caring for flock (5)
I believe the answer is:
swarm
'for flock' is the definition.
(I've seen this in another clue)
'shepherd firstly caring' is the wordplay.
'firstly' indicates taking the first letters.
'caring' becomes 'warm' (synonyms).
The initial letter of 'shepherd' is 's'.
's'+'warm'='SWARM'
(Other definitions for swarm that I've seen before include "Moving crowd, of bees say" , "Mob" , "Group of flies" , "Crowd of e.g. bees" , "Arrive in numbers" .)