Holidays including empty outdoor spas? (7)
I believe the answer is:
resorts
'spas?' is the definition.
(spa is a kind of resort)
'holidays including empty outdoor' is the wordplay.
'holidays' becomes 'rests' (I've seen this before).
'including' means one lot of letters goes inside another.
'empty' suggests removing the centre.
'outdoor' with its centre taken out is 'or'.
'rests' placed around 'or' is 'RESORTS'.
(Other definitions for resorts that I've seen before include "Holiday destinations" , "Haunts" , "places for holidays" , "Uses" , "Holiday places" .)