Indication of vice involving head of government (4)
I believe the answer is:
sign
'indication' is the definition.
(I know that sign is a type of indication)
'vice involving head of government' is the wordplay.
'vice' becomes 'sin'.
'involving' indicates putting letters inside.
'head of' says to take the initial letters.
The initial letter of 'government' is 'g'.
'sin' going around 'g' is 'SIGN'.
'of' is the link.
(Other definitions for sign that I've seen before include "Augury" , "Put one's name to a document" , "Engage by written agreement" , "Communicate to the deaf" , "Indication; symbol" .)