A detective follows unknown old South American (5)
I believe the answer is:
aztec
'old south american' is the definition.
'aztec' can be an answer for 'american' (Aztec is an example). I am unsure of the 'old south' bit.
'a detective follows unknown' is the wordplay.
'detective' becomes 'tec' (synonyms).
'follows' says to put letters next to each other.
'unknown' becomes 'z' (algebra).
'tec' after 'z' is 'ztec'.
'a'+'ztec'='AZTEC'
Can you help me to learn more?
(Other definitions for aztec that I've seen before include "Pre-Spanish Mexican" , "Old person" , "Pre-conquest Mexican Indian" , "old American" , "one of Mexican Indian extraction" .)