Resourceful, start to replace oxygen in passage (6)
I believe the answer is:
adroit
'resourceful' is the definition.
(I've seen this before)
'start to replace oxygen in passage' is the wordplay.
'start to' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'passage' becomes 'adit' (passage leading to a mine).
The initial letters of 'replace oxygen' is 'ro'.
'ro' placed inside 'adit' is 'ADROIT'.
(Other definitions for adroit that I've seen before include "Clever, skilful" , "Dexterous, artful" , "skill" , "Skilful, expert" , "Skilful, dexterous" .)