Bottom greeting is in the curve (6)
I believe the answer is:
behind
'bottom' is the definition.
(I know that bottom can be written as behind)
'greeting is in the curve' is the wordplay.
'greeting' becomes 'hi' (hi is a kind of greeting).
'is in the' indicates putting letters inside.
'curve' becomes 'bend' (bend is a kind of curve).
'hi' inserted inside 'bend' is 'BEHIND'.
(Other definitions for behind that I've seen before include "Astern" , "in van? No" , "Not up to date" , "Hidden by; bottom" , "Rear - losing" .)