Prevent chief becoming fool (9)
I believe the answer is:
blockhead
'fool' is the definition.
(thesaurus)
'prevent chief' is the wordplay.
'prevent' becomes 'block' (blocking is a kind of preventing**).
'chief' becomes 'head' ('head' can be a synonym of 'chief').
'block'+'head'='BLOCKHEAD'
'becoming' acts as a link.
(Other definitions for blockhead that I've seen before include "Dolt" , "Stupid person" .)