Urgent request from old man to imprison the French (4)
I believe the answer is:
plea
'urgent request' is the definition.
(I've seen this before)
'old man to imprison the french' is the wordplay.
'old man' becomes 'pa' (both can mean someone's father).
'to imprison' means one lot of letters goes inside another.
'the french' becomes 'le' ('the' in French).
'pa' going around 'le' is 'PLEA'.
'from' acts as a link.
(Other definitions for plea that I've seen before include "Lightly char" , "Request for mercy?" , "Formal statement of guilty or not guilty" , "Dispute" , "Intercession" .)