Hollow cheers reflected warning (6)
I believe the answer is:
caveat
'warning' is the definition.
(I know that caveat is a type of warning)
'hollow cheers reflected' is the wordplay.
'hollow' becomes 'cave' (caving is a kind of hollowing).
'cheers' becomes 'ta' (both can informally mean thank you).
'reflected' says the letters should be written backwards (letters reflected back).
'ta' backwards is 'at'.
'cave'+'at'='CAVEAT'
(Other definitions for caveat that I've seen before include "Vacate (anag) - warning" , "Reservation" , "Warning (of conditions)" , "A caution, proviso" , "Warning in a legal document" .)