Letters from mountain hospital: help! (8)
I believe the answer is:
alphabet
'letters' is the definition.
(thesaurus)
'mountain hospital help' is the wordplay.
'mountain' becomes 'alp' (alp is a kind of mountain).
'hospital' becomes 'h'.
'help' becomes 'abet' (I've seen this before).
'alp'+'h'+'abet'='ALPHABET'
'from' acts as a link.
(Other definitions for alphabet that I've seen before include "lots of characters" , "Ordered set of letters" , "All the characters together" , "series of letters" , "Group of characters" .)