Motivate last pair of men before besieging foreign fighters (8)
I believe the answer is:
energise
'motivate last' is the definition.
'energise' can be an answer for 'motivate' (I've seen this before). I'm unsure of the 'last' bit.
'pair of men before besieging foreign fighters' is the wordplay.
'pair of men' becomes 'en' (I can't justify this - if you can you should give a lot more credence to this answer).
'before' becomes 'ere' (usually literary).
'besieging' means one lot of letters goes inside another.
'foreign fighters' becomes 'gis' (GI can informally mean a US soldier).
'ere' placed around 'gis' is 'ergise'.
'en'+'ergise'='ENERGISE'
(Other definitions for energise that I've seen before include "give vigour" , "Give active force" , "Activate" , "To banish inertia" , "Motivate" .)