Base one? Good lord! (7)
I believe the answer is:
ignoble
'base' is the definition.
(similar in meaning)
'one? good lord' is the wordplay.
'one?' becomes 'I' (Roman numeral).
'good' becomes 'g' (abbreviation).
'lord' becomes 'noble' (synonyms).
'i'+'g'+'noble'='IGNOBLE'
(Other definitions for ignoble that I've seen before include "Not honourable in character or purpose" , "Unworthy" , "Cheap" , "Inferior" , "'Dishonourable, mean (7)'" .)