Indication of second unknown politician backing test (7)
I believe the answer is:
symptom
'indication' is the definition.
(I know that symptom is a type of indication)
'second unknown politician backing test' is the wordplay.
'second' becomes 's' (common abbreviation).
'unknown' becomes 'y' (algebra).
'politician' becomes 'MP' (Member of Parliament).
'backing' says the letters should be written backwards.
'test' becomes 'MOT' (MOT test).
'mot' in reverse letter order is 'tom'.
's'+'y'+'mp'+'tom'='SYMPTOM'
'of' is the link.
(Other definitions for symptom that I've seen before include "Troublesome sign" , "It indicates disease" , "Indicator of disease" , "It alerts one to illness" , "Sign of disease" .)