Small computer used in atomic rockets (5)
I believe the answer is:
micro
'small computer' is the definition.
(I've seen this before)
'in atomic rockets' is the wordplay.
'in' indicates the answer is hidden within the clue.
'MICRO' is hidden in the letters of 'atomic rockets'.
'used' is the link.
(Other definitions for micro that I've seen before include "Small computer" , "needed in kitchen-sink dramas?" , "minute" , "Extremely small" , "Prefix meaning tiny, or one-millionth of a unit" .)