Gaunt old American beneath fellow in saloon (10)
I believe the answer is:
cadaverous
'gaunt' is the definition.
(synonyms)
'old american beneath fellow in saloon' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'american' becomes 'us' (abbreviation for United States).
'beneath' says to put letters next to each other (in a down clue).
'fellow in saloon' becomes 'cadaver' (I can't justify this - if you can you should believe this answer much more).
'o'+'us'='ous'
'ous' put after 'cadaver' is 'CADAVEROUS'.
Can you help me to learn more?
(Other definitions for cadaverous that I've seen before include "Unhealthy-looking" , "Ghastly" , "not looking healthy" , "Corpse-like" , "Thin and bony like a corpse" .)