Mountaineer of the first order getting leg trapped (8)
I believe the answer is:
alpinist
'mountaineer' is the definition.
(alpinist is a kind of mountaineer)
'first order getting leg trapped' is the wordplay.
'first order getting' becomes 'alist' (I can't explain this - if you can you should give a lot more credence to this answer).
'leg' becomes 'pin' (pin is a kind of leg).
'trapped' indicates putting letters inside.
'alist' enclosing 'pin' is 'ALPINIST'.
'of the' is the link.
Can you help me to learn more?
(Other definitions for alpinist that I've seen before include "mountaineer" , "Maker of sharp climb" , "Climber of big mountains" , "one climbs" .)