Ex-PM in wrongdoing, needing good protection (9)
I believe the answer is:
sheathing
'protection' is the definition.
(I know that sheathing is a type of protection)
'ex-pm in wrongdoing needing good' is the wordplay.
'ex-pm' becomes 'heath' (prime minister Edward Heath).
'in' indicates putting letters inside.
'wrongdoing' becomes 'sin' (sin is a kind of wrongdoing).
'needing' means one lot of letters go next to another.
'good' becomes 'g' (abbreviation).
'heath' going inside 'sin' is 'sheathin'.
'sheathin'+'g'='SHEATHING'
(Other definitions for sheathing that I've seen before include "putting it in a case" , "protective covering" .)