Military commander to be given leadership of regiment in just over four weeks (7)
I believe the answer is:
october
'just over four weeks' is the definition.
'military commander to be given leadership of regiment' is the wordplay.
'military commander' becomes 'oc' (abbreviation for officer commanding).
'be given' means one lot of letters go next to another.
'leadership of regiment' becomes 'ber' (I can't explain this - if you can you should believe this answer much more).
'oc'+'to'+'ber'='OCTOBER'
'in' acts as a link.
(Other definitions for october that I've seen before include "some weeks" , "To be orc (anag)" , "The Hunt for Red . . . . . . . (film)" , "21st = Trafalgar Day" , "Eighth month on the Roman calendar, originally" .)