Dead calm at last in centre (4)
I believe the answer is:
numb
'dead' is the definition.
(thesaurus)
'calm at last in centre' is the wordplay.
'at last' says to take the final letters.
'in' is an insertion indicator.
'centre' becomes 'nub' (I've seen this before**).
The last letter of 'calm' is 'm'.
'm' inserted inside 'nub' is 'NUMB'.
(Other definitions for numb that I've seen before include "Deadened, desensitised" , "Stunned, insensible" , "Lacking feeling" , "Feeling nothing" , "Without feeling" .)