Island MP has easy win — trade union unprepared (9)
I believe the answer is:
'unprepared' is the definition.
(impromptu means improvised or unprepared)
'island mp has easy win trade union' is the wordplay.
'island' becomes 'i' (geographical abbreviation).
'has' means one lot of letters go next to another.
'easy win' becomes 'romp' (I've seen this in another clue).
'trade union' becomes 'TU' (abbreviation for trade union).
(Other definitions for impromptu that I've seen before include "Ad hoc: spontaneous-sounding musical piece mastered by Schubert" , "On the spur of the moment" , "Ad-lib" , "Unplanned; instrumental piece" , "Unscripted" .)