Noel's doctor rings Mike - it's grave (6)
I believe the answer is:
solemn
'grave' is the definition.
('solemn' can be a synonym of 'grave')
'noel's doctor rings mike' is the wordplay.
'doctor' is an anagram indicator.
'rings' is an insertion indicator.
'mike' becomes 'm' (phonetic alphabet: alpha, bravo, charlie etc.).
'noels' is an anagram of 'solen'.
'solen' going around 'm' is 'SOLEMN'.
'it's' is the link.
(Other definitions for solemn that I've seen before include "Formal, ceremonius" , "Marked by pomp, cermony or formality" , "Unsmiling" , "Formal, dignified" , "Stately, pompous or glum" .)