Doing a deviation, relative to identification (9)
I believe the answer is:
diagnosis
'to identification' is the definition.
(diagnosis is a kind of identification)
'doing a deviation relative' is the wordplay.
'deviation' indicates anagramming the letters.
'relative' becomes 'sis' (short for sister).
'doing'+'a'='doinga'
'doinga' with letters rearranged gives 'diagno'.
'diagno'+'sis'='DIAGNOSIS'
(Other definitions for diagnosis that I've seen before include "Identification of an illness" , "medical opinion" , "analysis" , "doctor's opinion" , "Identification of a problem from its symptoms" .)