Charge a deserter at mid-September (4)
I believe the answer is:
rate
'charge' is the definition.
(I know that rate is a type of charge)
'a deserter at mid-september' is the wordplay.
'a deserter' becomes 'rat' (both can mean a traitor).
'at' says to put letters next to each other.
'mid' means to look at the middle letters (mid- can mean the middle of something).
'september' becomes 'sep' (short for September).
The centre of 'sep' is 'e'.
'rat'+'e'='RATE'
(Other definitions for rate that I've seen before include "Tear off, gaining speed" , "Consider" , "Measure as of value or speed" , "The speed of movement" , "Estimate or deserve" .)