Record kept by geologist (3)
I believe the answer is:
log
'record' is the definition.
(I know that log is a type of record)
'kept by geologist' is the wordplay.
'kept by' indicates the answer is hidden within the clue.
'LOG' is hidden in the letters of 'geologist'.
(Other definitions for log that I've seen before include "Record; cut wood" , "number in maths class?" , "number in mathematical table" , "fuel consumed" , "Register" .)