Keen copper coming in had meal (5)
I believe the answer is:
acute
'keen' is the definition.
(I know that keen can be written as acute)
'copper coming in had meal' is the wordplay.
'copper' becomes 'Cu' (chemical symbol).
'coming in' means one lot of letters goes inside another.
'had meal' becomes 'ate' (to have a meal is to eat).
'cu' inserted into 'ate' is 'ACUTE'.
(Other definitions for acute that I've seen before include "Perspicacious" , "Critical, serious" , "Sharp; severe" , "Discerning" , "Urgently pressing" .)