What's up with greeting cast? (5)
I believe the answer is:
heave
'cast?' is the definition.
(heaving is a kind of casting)
'what's up with greeting' is the wordplay.
'what' becomes 'eh' (sound used to ask 'what?' or 'pardon?').
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'with' is a charade indicator (letters next to each other).
'greeting' becomes 'ave' (Latin greeting).
'eh' written backwards gives 'he'.
'he'+'ave'='HEAVE'
(Other definitions for heave that I've seen before include "Express" , "Struggle to move" , "Throw with force - retch" , "Throw, hurl" , "Pull - retch" .)