Investigated how party leader dressed (6)
I believe the answer is:
probed
'investigated' is the definition.
(probing is a kind of investigating)
'party leader dressed' is the wordplay.
'party leader' becomes 'p' (leader of the word 'party').
'dressed' becomes 'robed' (synonyms).
'p'+'robed'='PROBED'
'how' acts as a link.
(Other definitions for probed that I've seen before include "Did research" , "Investigated closely as dentist does a tooth" , "Physically examined" , "Questioned or examined thoroughly and closely" , "Investigating closely" .)