Clue: 20 parts, 2 held by climbers, 1 by mountaineers (9)
Answer: ASCENDERS
Explanation: Definition: 20 parts
An ascender is part of a letter that extends above the middle (i.e. above the height of letters such as n and a). It can also refer to a letter that has an ascender. The words "climbers" and "mountaineers" have two (l, b) and one (t) of these respectively (and climbers and mountaineers are also "ascenders", though that doesn't seem to be a necessary part of the clue)