Fool's gold found in second excavation, finally (5)
I believe the answer is:
moron
'fool's' is the definition.
(thesaurus)
'gold found in second excavation finally' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'found in' is an insertion indicator.
'second' becomes 'mo' (both are short periods of time).
'finally' says to take the final letters.
The final letter of 'excavation' is 'n'.
'or' going inside 'mo' is 'moro'.
'moro'+'n'='MORON'
(Other definitions for moron that I've seen before include "Wally" , "Simpleton" , "Person of subnormal intelligence" , "Halfwit" , "Idiot" .)