Ace investigator pens last letter for old American (5)
I believe the answer is:
aztec
'old american' is the definition.
(I've seen this before)
'ace investigator pens last letter' is the wordplay.
'ace' becomes 'a' (abbreviation).
'investigator' becomes 'tec' (short for 'detective').
'pens' is an insertion indicator.
'last letter' becomes 'z' (last letter of the alphabet).
'a'+'tec'='atec'
'atec' placed around 'z' is 'AZTEC'.
'for' acts as a link.
(Other definitions for aztec that I've seen before include "Like Montezuma?" , "One into human sacrifice" , "Original Mexican native" , "Former Mexican - Indian race" , "Old person" .)