Visiting Namibia on vacation, ordered blue Crab for one (6)
I believe the answer is:
nebula
'crab for one' is the definition.
(as in the Crab Nebula in our solar system)
'visiting namibia on vacation ordered blue' is the wordplay.
'visiting' means one lot of letters goes inside another.
'on vacation' says to hollow out the word (remove centre letters) (after the inside letters have vacated the word).
'ordered' indicates anagramming the letters.
'namibia' with its middle taken out is 'na'.
'blue' with letters rearranged gives 'ebul'.
'na' placed around 'ebul' is 'NEBULA'.
(Other definitions for nebula that I've seen before include "Misty appearance in the heavens" , "Gas/dust cloud in space" , "Diffuse cloud of particles and gas in space" , "Cosmic cloud" , "Bright patch in sky caused by distant galaxy" .)