Dead centre describes 'middle', principally (4)
I believe the answer is:
numb
'dead' is the definition.
(thesaurus)
'centre describes middle principally' is the wordplay.
'centre' becomes 'nub' (I've seen this before**).
'describes' is an insertion indicator (describe can mean to form the outline of).
'principally' indicates taking the first letters.
The initial letter of 'middle' is 'm'.
'nub' enclosing 'm' is 'NUMB'.
(Other definitions for numb that I've seen before include "Dead" , "Deprived of sensation" , "Deaden" , "Stinks" , "Stunned, insensible" .)