"Retreat," soldiers heading to the front communicated silently (6)
I believe the answer is:
signed
'communicated silently' is the definition.
'signed' can be an answer for 'communicated' (signing is a kind of communicating). I am not sure about the 'silently' bit.
'retreat soldiers heading to the front' is the wordplay.
'retreat' shows that the letters should be reversed in order.
'soldiers' becomes 'GIs' (GI is an informal term for a US soldier).
'heading' says to put letters next to each other (some letters go in front of or head others).
'to the front' becomes 'ned' (I can't explain this - if you can you should give a lot more credence to this answer).
'gis' back-to-front is 'sig'.
'sig'+'ned'='SIGNED'
Can you help me to learn more?
(Other definitions for signed that I've seen before include "Wrote one's name" , "Inscribed - endorsed" , "Put one's name to" , "Authorised (in writing)" , "Autographed" .)