Backing positive, steadfast youth leaders to undertake major journey (7)
I believe the answer is:
odyssey
'journey' is the definition.
(I know that odyssey is a type of journey)
'backing positive steadfast youth leaders to undertake major' is the wordplay.
'backing' shows that the letters should be reversed in order.
'positive' becomes 'yes'.
'leaders' indicates taking the first letters.
'to undertake major' becomes 'do' (I can't justify this - if you can you should give a lot more credence to this answer).
The first letters of 'steadfast youth' is 'sy'.
'yes'+'sy'+'do'='yessydo'
'yessydo' reversed gives 'ODYSSEY'.
Can you help me to learn more?
(Other definitions for odyssey that I've seen before include "Long, eventful, wandering journey" , "Homer's epic poen about great journey" , "Epic poem - epic voyage" , "A long adventurous, as told by Homer" , "Greek epic on the wanderings of Ulysses" .)