Old Spaniard in firm runs trial that's incomplete (6)
I believe the answer is:
cortes
'old spaniard' is the definition.
The definition suggests a singular noun which matches the answer.
'firm runs trial that's incomplete' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'runs' becomes 'r' (cricket abbreviation).
'trial' becomes 'test' ('test' can be a synonym of 'trial').
'that's incomplete' means to remove the last letter.
'test' with its final letter removed is 'tes'.
'co'+'r'+'tes'='CORTES'
'in' is the link.
Can you help me to learn more?
(Other definitions for cortes that I've seen before include "Spanish conquistador who overthrew the Aztec emperor Montezuma, d. 1547" , "Spanish conquistador, d.1547" , "Conquering Spaniard" , "Spanish parliament, sector (anag.)" .)