Missile starts to seem costly -- unwanted deterrent? (4)
I believe the answer is:
scud
'missile' is the definition.
(I've seen this before)
'starts to seem costly unwanted deterrent?' is the wordplay.
'starts to' says to take the initial letters.
The first letters of 'seem costly unwanted deterrent' is 'SCUD'.
(Other definitions for scud that I've seen before include "Move swiftly in the wind" , "A cloud might do it" , "Ballistic missile; sweep along" , "Move along smoothly - run before gale - Russian missile" , "Move along quickly and smoothly" .)