Made uninvited use of resorts in the elevated Kalahari, perhaps (10)
I believe the answer is:
trespassed
'made uninvited use of' is the definition.
(to enter somewhere without permission)
'resorts in the elevated kalahari perhaps' is the wordplay.
'resorts' becomes 'spas' (spa is a kind of resort).
'in' indicates putting letters inside.
'the elevated' is a reversal indicator (down clues only).
'kalahari perhaps' becomes 'desert' (kalahari is a kind of desert).
'desert' written backwards gives 'tresed'.
'spas' going within 'tresed' is 'TRESPASSED'.
(Another definition for trespassed that I've seen is " had no right to be there".)