Former star to boycott part of course (5,4)
I believe the answer is:
black hole
'former star' is the definition.
(a black hole is formed by a collapsing star)
'boycott part of course' is the wordplay.
'boycott' becomes 'black' (black can mean to refuse to handle certain goods).
'part of course' becomes 'hole' (hole on a golf course).
'black'+'hole'='BLACK HOLE'
'to' acts as a link.
(Other definitions for black hole that I've seen before include "Astronomical curiosity" , "discovered in Calcutta" , "object in 21 [SPACE] having very strong 20 [GRAVITY]" , "Space left after collapse of a star" , "'Region of space from which nothing can escape (5,4)'" .)