Just to see the king rocket? (4)
I believe the answer is:
ajar
'just to' is the definition.
(both can mean 'slightly open')
'the king rocket?' is the wordplay.
'the king' becomes 'raja' (historical term for an Indian king or prince).
'rocket?' says the letters should be written in reverse (letters go up, in a down clue).
'raja' reversed gives 'AJAR'.
'see' acts as a link.
(Other definitions for ajar that I've seen before include "not entirely open" , "Half open" , "not slammed shut" , "opened tentatively?" , "Open by a chink" .)