Crossword Genius

Principal drive ratio creates bonnet, say (8)

Ross

I believe the answer is:

headgear

I'm a little stuck... Click here to teach me more about this clue!

'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?" .)

I've seen this clue in the Sydney Morning Herald.
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