Older person dismissing one Spanish fellow (5)
I believe the answer is:
senor
'spanish fellow' is the definition.
(equivalent to 'mister' in Spanish)
'older person dismissing one' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'one' is 'eno' which is within the answer.
'older' could be 'sr' (synonyms) and 'sr' is found in the leftover letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for senor that I've seen before include "Norse (anag.)" , "'Mister' in Spain" , "Snore like Spaniard" , "Gentlemanly address in Spain" , "Spanish gent of Norse origin" .)