Formerly you have prophetic sign for British farmers (6)
I believe the answer is:
yeomen
'farmers' is the definition.
(I have seen 'farmers of old' mean 'yeomen' so perhaps 'farmers' could also mean 'yeomen')
'formerly you have prophetic sign for british' is the wordplay.
I cannot quite see how this works, but
'you' could be 'ye' (historical version of 'you') and 'ye' is located in the answer.
'sign' could be 'omen' (omen is a kind of sign) and 'omen' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for yeomen that I've seen before include "Freeholders (historical)" , "Freeholders (hist.)" , "Guards" , "Petty officers" , "farmers of old" .)