Principal drive ratio creates bonnet, say (8)
I believe the answer is:
headgear
'bonnet say' is the definition.
(I know that poke bonnet is a type of hat)
'principal drive ratio' is the wordplay.
'principal' becomes 'head' (synonyms).
'drive ratio' becomes 'gear' (I can't explain this - if you can you should believe this answer much more).
'head'+'gear'='HEADGEAR'
'creates' acts as a link.
Can you help me to learn more?
(Other definitions for headgear that I've seen before include "Perhaps a hat" , "crown, say" , "Balmoral, perhaps" , "Aged hare? (anagram)" , "which sits on top of mine?" .)