Courageous favourite length adrift (6)
I believe the answer is:
daring
'courageous' is the definition.
(daring can mean adventurous or courageous)
'favourite length adrift' is the wordplay.
'favourite' becomes 'darling' (synonyms).
'length' becomes 'l' (abbreviation in geometry).
'adrift' suggests deleting specific letters.
'darling' with 'l' taken away is 'DARING'.
(Other definitions for daring that I've seen before include "Venturesome" , "Audacious - bravery" , "Risky" , "Impudent - brave - courage" , "'Adventurous, brave (6)'" .)