Principal part of transmission of caps, say (8)
I believe the answer is:
headgear
'caps say' is the definition.
(I know that cap is a type of headdress)
'principal part of transmission' is the wordplay.
'principal' becomes 'head' (synonyms).
'part of transmission' becomes 'gear' (I can't explain this - if you can you should believe this answer much more).
'head'+'gear'='HEADGEAR'
'of' is the link.
Can you help me to learn more?
(Other definitions for headgear that I've seen before include "Balmoral, perhaps" , "which sits on top of mine?" , "bowler perhaps" , "Hats, bonnets, caps etc" , "Perhaps a hat" .)