Scale in affluent bacterial centre (7)
I believe the answer is:
richter
'scale' is the definition.
(Richter scale is an example)
'affluent bacterial centre' is the wordplay.
'affluent' becomes 'rich' (similar in meaning).
'centre' says to take the centre.
The central letters of 'bacterial' are 'ter'.
'rich'+'ter'='RICHTER'
'in' acts as a link.
(Other definitions for richter that I've seen before include "Seismologist" , "Physicist" , "Scale for measuring earth tremors" , "Earthquakes are measured on this scale" , "Scale on which earth tremors are measured" .)