Small ballistic missile, shrill (8)
I believe the answer is:
strident
'shrill' is the definition.
(thesaurus)
'small ballistic missile' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'ballistic missile' becomes 'trident' (I've seen this before).
's'+'trident'='STRIDENT'
(Other definitions for strident that I've seen before include "Clamorous" , "Commanding attention" , "Loud and harsh over tridents" , "Loudly assertive" , "Tumultuous" .)