The most advanced military units are in direct contact with the public (5,4)
I believe the answer is:
front line
'the public' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'the most advanced military units are in direct contact' is the wordplay.
I cannot really understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
'in' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'with' is the link.
Can you help me to learn more?
(Other definitions for front line that I've seen before include "Where the fighting takes place" , "sharp end" , "Most advanced troops" .)