Top gear selected by a duchess perhaps (5)
I believe the answer is:
tiara
'top gear selected' is the definition.
I can't tell whether this defines the answer.
'by a duchess perhaps' is the wordplay.
'by' says to put letters next to each other.
'duchess perhaps' becomes 'tiar' (I can't justify this - if you can you should give a lot more credence to this answer).
'a' put after 'tiar' is 'TIARA'.
Can you help me to learn more?
(Other definitions for tiara that I've seen before include "special headgear" , "Jewelled head band" , "Jewelled head ornament" , "Woman's head ornament" , "It bejewels" .)