Small missile system thundering (8)
I believe the answer is:
strident
'thundering' is the definition.
'small missile system' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'missile system' becomes 'trident' (UK missile system).
's'+'trident'='STRIDENT'
(Other definitions for strident that I've seen before include "With a loud, harsh voice" , "Commanding attention" , "giving views forcefully" , "Noisy" , "Unpleasantly loud and harsh" .)