Dine on steak, say, but not medium (3)
I believe the answer is:
eat
'dine on' is the definition.
(I've seen this in another clue)
'steak say but not medium' is the wordplay.
'steak say' becomes 'meat' (steak is a kind of meat).
'but not' indicates named letters should be taken away.
'medium' becomes 'm' (as in UK clothing sizes).
'meat' with 'm' taken out is 'EAT'.
(Other definitions for eat that I've seen before include "Bit, chew and swallow" , "and 27: Be humiliated" , "Be corrosive" , "Ingest" , "Bite, chew and swallow" .)