Rocket stage (7)
I believe the answer is:
booster
'rocket stage' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for booster that I've seen before include "Additional strengthening dose" , "Space rocket's first stage" , "tonic" , "Additional dose that makes sure the first was effective" , "Additional dose to make sure the first one was effective" .)