Vetting, at sea, a French pontoon (5-2-2)
I believe the answer is:
vingt-et-un
'pontoon' is the definition.
(I've seen this before)
'vetting at sea a french' is the wordplay.
'at sea' is an anagram indicator.
'a french' becomes 'un' ('a' in French).
'vetting' anagrammed gives 'vingtet'.
'vingtet'+'un'='VINGT-ET-UN'
(Other definitions for vingt-et-un that I've seen before include "Pontoon (French)" , "Pontoon, or blackjack" .)