A reason for living or, in a desert, moving (6,5)
I believe the answer is:
raison detre
'a reason for living' is the definition.
(I've seen this before)
'or in a desert moving' is the wordplay.
'moving' indicates anagramming the letters.
'or'+'in'+'a'+'desert'='orinadesert'
'orinadesert' anagrammed gives 'RAISON DETRE'.
(Other definitions for raison detre that I've seen before include "Reason for existence (6,1'4)" , "Justification for existence" , "Reason for living" , "Why one is" , "Purpose of existence" .)