Missile so clued occasionally (4)
I believe the answer is:
scud
'missile' is the definition.
(type of long-range missile)
'so clued occasionally' is the wordplay.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'so'+'clued'='soclued'
The alternating letters of 'soclued' are 'SCUD'.
(Other definitions for scud that I've seen before include "Move fast (like a cloud?)" , "Russian ballistic missile" , "swift runner" , "Move along smoothly - run before gale - Russian missile" , "Traverse swiftly" .)