Whip's greeting included warning (6)
I believe the answer is:
caveat
'warning' is the definition.
(I know that caveat is a type of warning)
'whip's greeting included' is the wordplay.
'whip' becomes 'cat' (cat o' nine tails, cat for short is a type of whip).
'greeting' becomes 'ave' (Latin greeting).
'included' means one lot of letters goes inside another.
'cat' going around 'ave' is 'CAVEAT'.
(Other definitions for caveat that I've seen before include "Let the buyer beware ...emptor" , "A notice or warning" , "A caution, proviso" , "Vacate (anag) - warning" , "A warning against certain acts" .)