Cast's brief relationship (5)
I believe the answer is:
fling
'relationship' is the definition.
(I have seen 'Brief relationship' mean 'fling' so perhaps 'relationship' could also mean 'fling')
'cast's brief' is the wordplay.
'cast's' becomes 'fling's' (both can mean to throw).
'brief' means to remove the last letter (a shorter or briefer version of the word).
'flings' with its last letter taken off is 'FLING'.
(Other definitions for fling that I've seen before include "Throw angrily in spell of indulgent pleasure" , "extramarital affair" , "Throw as in Highland dance?" , "Throw, cast" , "Throw - short affair" .)