Serious in conclusion — deeply impressed? (8)
I believe the answer is:
engraved
'deeply impressed?' is the definition.
(I've seen this before)
'serious in conclusion' is the wordplay.
'serious' becomes 'grave' (both can mean solemn).
'in' indicates putting letters inside.
'conclusion' becomes 'end' (something's conclusion is its end).
'grave' put within 'end' is 'ENGRAVED'.
(Other definitions for engraved that I've seen before include "deeply impressed" , "Carved into surface" , "Cut or impressed into a surface" , "Cut, carved (a design)" .)