Seismologist's unfinished biscuit beginning to rot (7)
I believe the answer is:
richter
'seismologist's' is the definition.
(Charles Francis Richter devised the Richter scale)
'unfinished biscuit beginning to rot' is the wordplay.
'unfinished' means to remove the last letter.
'biscuit' becomes 'rich tea'.
'beginning to' suggests taking the first letters.
The first letter of 'rot' is 'r'.
'richtea' with its last letter taken away is 'richte'.
'richte'+'r'='RICHTER'
(Other definitions for richter that I've seen before include "Scale on which earth tremors are measured" , "Earthquakes are measured on this scale" , "scale of upheavals" , "Seismological scale" , "Physicist" .)