Boosts soldiers after a month, way up (8)
I believe the answer is:
augments
'boosts' is the definition.
(to augment is to boost or add to)
'soldiers after a month way up' is the wordplay.
'soldiers' becomes 'men' (I've seen this before).
'after' means one lot of letters go next to another.
'a month' becomes 'Aug' (abbreviation for August).
'way up' becomes 'ts' (I can't explain this - if you can you should give a lot more credence to this answer).
'men' after 'aug' is 'augmen'.
'augmen'+'ts'='AUGMENTS'
(Other definitions for augments that I've seen before include "Enlarges or increases" , "Adds to, increases" , "Extends" , "Stun game makes larger" .)