Piercing, small missile (8)
I believe the answer is:
strident
'piercing' is the definition.
(thesaurus)
'small missile' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'missile' becomes 'Trident' (UK nuclear weapons system).
's'+'trident'='STRIDENT'
(Other definitions for strident that I've seen before include "Commanding attention" , "With a loud, harsh voice" , "Clamorous" , "making loud noises" , "harsh noise" .)