Politician in a flash becoming unreliable (10)
I believe the answer is:
inconstant
'unreliable' is the definition.
(thesaurus)
'politician in a flash' is the wordplay.
'politician' becomes 'con'.
'in' is an insertion indicator.
'a flash' becomes 'instant' ('instant' can be a synonym of 'flash'**).
'con' going into 'instant' is 'INCONSTANT'.
'becoming' is the link.
(Other definitions for inconstant that I've seen before include "Fickle" , "Often changing" , "Variable, irregular" .)