Mountaineer has an inclination to swallow a peg (8)
I believe the answer is:
alpinist
'mountaineer' is the definition.
(alpinist is a kind of mountaineer)
'an inclination to swallow a peg' is the wordplay.
'an' becomes 'a'.
'inclination' becomes 'list' (list can mean to tilt, lean or incline).
'to swallow' means one lot of letters goes inside another.
'a peg' becomes 'pin' ('pin' can be a synonym of 'peg').
'list' placed around 'pin' is 'lpinist'.
'a'+'lpinist'='ALPINIST'
'has' acts as a link.
(Other definitions for alpinist that I've seen before include "Maker of sharp climb" , "Climber of big mountains" , "one climbs" , "mountaineer" .)