Legion assembled around British base (7)
I believe the answer is:
ignoble
'base' is the definition.
('ignoble' can be similar in meaning to 'base')
'legion assembled around british' is the wordplay.
'assembled' indicates anagramming the letters.
'around' indicates putting letters inside.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'legion' with letters rearranged gives 'ignole'.
'ignole' going around 'b' is 'IGNOBLE'.
(Other definitions for ignoble that I've seen before include "Contemptible" , "'Base, dishonourable (7)'" , "in a lower category" , "Wretched, base" , "Debased in character or purpose" .)