Bronze age tin surprisingly left off the main agenda (10)
I believe the answer is:
tangential
'off the main agenda' is the definition.
I can't judge whether this defines the answer.
'bronze age tin surprisingly left' is the wordplay.
'bronze' becomes 'tan' (both can mean to get a sun tan).
'surprisingly' is an anagram indicator (a surprising ordering of the letters).
'left' becomes 'l' (common abbreviation).
'age'+'tin'='agetin'
'agetin' anagrammed gives 'gentia'.
'tan'+'gentia'+'l'='TANGENTIAL'
Can you help me to learn more?
(Other definitions for tangential that I've seen before include "Peripheral" , "Irrelevant" .)