Lacking confidence in Belgium's premier, a busted flush (7)
I believe the answer is:
bashful
'lacking confidence' is the definition.
(I've seen this before)
'belgium's premier a busted flush' is the wordplay.
'premier' says to take the initial letters.
'busted' indicates an anagram.
The first letter of 'belgium' is 'b'.
'flush' is an anagram of 'shful'.
'b'+'a'+'shful'='BASHFUL'
'in' is the link.
(Other definitions for bashful that I've seen before include "Neither Happy, Sneezy, Dopey, Grumpy, Sleepy nor Doc!" , "Lacking confidence, self-conscious" , "reserved" , "One of Disney's dwarves" , "Shy, timid" .)