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' indicates putting letters inside.
'on vacation' means to remove the middle letters (after the inside letters have vacated the word).
'ordered' is an anagram indicator.
'namibia' with its middle removed is 'na'.
'blue' with letters rearranged gives 'ebul'.
'na' enclosing '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" .)