Cast the short line (5)
I believe the answer is:
throw
'cast' is the definition.
('throw' can be a synonym of 'cast')
'the short line' is the wordplay.
'short' means to remove the last letter.
'line' becomes 'row' (row is a kind of line**).
'the' with its last letter taken off is 'th'.
'th'+'row'='THROW'
(Other definitions for throw that I've seen before include "A turn with the dice" , "Cast; give (party)" , "Light rug" , "Chuck - confuse" , "Organise party" .)