Son enters competition to make point (4)
I believe the answer is:
cusp
'point' is the definition.
(cusp is a kind of point)
'son enters competition' is the wordplay.
'son' becomes 's'.
'enters' is an insertion indicator.
'competition' becomes 'cup' (I've seen this before).
's' put into 'cup' is 'CUSP'.
'to make' acts as a link.
(Other definitions for cusp that I've seen before include "Point where two curves meet - point of transition" , "One point of a crescent moon" , "Pointed end - division between signs of the zodiac" , "Turning point on which important developments depend" , "Point (e.g. of tooth)" .)