One telling soldiers to retreat on plain (7)
I believe the answer is:
utterer
'one telling' is the definition.
Although both the answer and 'telling' are adjectives, they are not in the same form of the adjective.
'soldiers to retreat on plain' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'to retreat' shows that the letters should be reversed in order.
'on' indicates putting letters inside.
'plain' becomes 'utter' (I can't explain this - if you can you should give a lot more credence to this answer).
're' backwards is 'er'.
'er' going within 'utter' is 'UTTERER'.
Can you help me to learn more?
(Other definitions for utterer that I've seen before include "Speaker" , "Person speaking" , "Person who announces" , "One speaks" .)