Foremost character, first in expedition, probing mountain height (5)
I believe the answer is:
aleph
'foremost character' is the definition.
'aleph' can be an answer for 'character' (I've seen this before). I'm not sure about the 'foremost' bit.
'first in expedition probing mountain height' is the wordplay.
'first in expedition' becomes 'e' (1st letter of 'expedition').
'probing' is an insertion indicator (probe can mean to make a hole in).
'mountain' becomes 'alp' (the Alps are a mountain range).
'height' becomes 'h' (abbreviation in geometry).
'e' placed within 'alp' is 'alep'.
'alep'+'h'='ALEPH'
Can you help me to learn more?
(Other definitions for aleph that I've seen before include "First letter of the Hebrew alphabet" , "letter in Middle East" , "Hebrew character" , "First character" , "Abroad, a" .)