Sappers leaving one French city for base (7)
I believe the answer is:
ignoble
'for base' is the definition.
(both can mean dishonourable or shameful)
'sappers leaving one french city' is the wordplay.
'sappers' becomes 'RE' (nickname for the Royal Engineers).
'leaving' suggests deleting specific letters.
'one' becomes 'i' (Roman numeral).
'french city' becomes 'grenoble' (city in southeastern France).
'i'+'grenoble'='igrenoble'
'igrenoble' with 're' taken out is 'IGNOBLE'.
(Other definitions for ignoble that I've seen before include "Humble; despicable" , "Contemptible" , "Inferior" , "Wretched, base" , "'Far from the madding crowd's . . . . . . . strife'" .)