National emblem that's terrifying on lieutenant's face (4)
I believe the answer is:
leek
'national emblem' is the definition.
(I've seen this before)
'terrifying on lieutenant's face' is the wordplay.
'terrifying' becomes 'eek' (I can't explain this - if you can you should believe this answer much more).
'on' is a charade indicator (letters next to each other).
'face' says to take the initial letters.
The initial letter of 'lieutenants' is 'l'.
'eek' put after 'l' is 'LEEK'.
'that's' is the link.
Can you help me to learn more?
(Other definitions for leek that I've seen before include "Vegetable associated with Wales" , "Plant (onion relative) eaten as a vegetable" , "Welsh national symbol" , "Eat the vegetable and keel over" , "Green and white vegetable" .)