A missile from Uncle (3)
I believe the answer is:
sam
I believe this clue is a double definition.
'a missile' is the first definition.
(abbreviation for surface-to-air missile)
'uncle' is the second definition.
(Uncle Sam is an example)
'from' acts as a link.
(Other definitions for sam that I've seen before include "Casablanca piano player" , "Boy" , "together once" , "Uncle" .)