Flier takes first adventurous six over a peak (7)
I believe the answer is:
aviator
'flier' is the definition.
(synonyms)
'first adventurous six over a peak' is the wordplay.
'first adventurous' becomes 'a' (1st letter of 'adventurous').
'six' becomes 'VI' (roman numeral).
'over' means one lot of letters go next to another.
'peak' becomes 'tor' (I've seen this before).
'a'+'vi'+'a'+'tor'='AVIATOR'
'takes' is the link.
(Other definitions for aviator that I've seen before include "One flying aircraft" , "Aircraft pilot" , "Aeronaut" , "Flyer, pilot" , "Pilot; style of sunglasses" .)