Drove Sierra close (5)
I believe the answer is:
swarm
'drove' is the definition.
(synonyms)
'sierra close' is the wordplay.
'sierra' becomes 'S' (phonetic alphabet: alpha, bravo, charlie etc.).
'close' becomes 'warm' (similar in meaning).
's'+'warm'='SWARM'
(Other definitions for swarm that I've seen before include "Host" , "Hive" , "Large cloud (of insects)" , "Mass of bees" , "Dense cloud (of insects)" .)