Unhappy after short vacation in cave (6)
I believe the answer is:
hollow
'cave' is the definition.
(thesaurus)
'unhappy after short vacation' is the wordplay.
'unhappy' becomes 'low'.
'after' means one lot of letters go next to another.
'short' means to remove the last letter.
'vacation' becomes 'hols' (I've seen this before).
'hols' with its final letter taken off is 'hol'.
'low' put after 'hol' is 'HOLLOW'.
'in' is the link.
(Other definitions for hollow that I've seen before include "Not solid, having hole in the middle" , "Concave - sunken" , "Empty in the middle" , "Empty - hole" , "Empty, not solid" .)