One group of soldiers abandoning French city base (7)
I believe the answer is:
ignoble
'base' is the definition.
(both can mean dishonourable or shameful)
'one group of soldiers' is the wordplay.
'one' becomes 'i' (Roman numeral).
'group of' becomes 'G' (eg the G7 nations, Group of Seven).
'soldiers' becomes 'noble' (I can't explain this - if you can you should believe this answer much more).
'i'+'g'+'noble'='IGNOBLE'
'abandoning french city' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for ignoble that I've seen before include "Contemptible" , "degenerate" , "Lacking dignity" , "'Dishonourable, mean (7)'" , "'Far from the madding crowd's . . . . . . . strife'" .)