Gold coin, second of many found in Channel (5)
I believe the answer is:
ducat
'gold coin' is the definition.
(historical coin of Europe)
'second of many found in channel' is the wordplay.
'second of many' becomes 'a' (und letter of 'many').
'found in' indicates putting letters inside.
'channel' becomes 'duct' (synonyms).
'a' inserted inside 'duct' is 'DUCAT'.
(Other definitions for ducat that I've seen before include "Money no longer" , "Mad; fruit" , "piece of gold" , "Old Italian coin" , "Old gold coin in e.g. Venice" .)