Robot starts to design rocket on its doorstep (5)
I believe the answer is:
droid
'robot' is the definition.
(I have seen 'small robot' mean 'droid' so perhaps 'robot' could also mean 'droid')
'starts to design rocket on its doorstep' is the wordplay.
'starts to' suggests taking the first letters.
The first letters of 'design rocket on its doorstep' is 'DROID'.
(Other definitions for droid that I've seen before include "small robot" , "automaton" .)