Rest the French in a month (5)
I believe the answer is:
sleep
'rest' is the definition.
(sleeping is a kind of resting)
'the french in a month' is the wordplay.
'the french' becomes 'le'.
'in' is an insertion indicator.
'a month' becomes 'sep' (Sep is an example**).
'le' placed within 'sep' is 'SLEEP'.
(Other definitions for sleep that I've seen before include "Drop off" , "Produced (egg)" , "''... that knits up the ravell'd sleeve of care'' (Macbeth)" , "crash" , "Doze, nap" .)