Boycott lecturer in the end (5)
I believe the answer is:
black
'boycott' is the definition.
(black can mean to refuse to handle certain goods)
'lecturer in the end' is the wordplay.
'lecturer' becomes 'l' (abbreviation for lecturer).
'in' is an insertion indicator.
'the end' becomes 'back' (back can mean the end of something).
'l' going within 'back' is 'BLACK'.
(Other definitions for black that I've seen before include "(Of coffee) without milk" , "Unofficial" , "Gloomy" , "Like clubs and spades" , "second player on board" .)