Cast most of the line (5)
I believe the answer is:
throw
'cast' is the definition.
('throw' can be a synonym of 'cast')
'most of the line' is the wordplay.
'most of' means to remove the last letter.
'line' becomes 'row' (row is a kind of line).
'the' with its final letter taken away is 'th'.
'th'+'row'='THROW'
(Other definitions for throw that I've seen before include "Cast; give (party)" , "Discard" , "Try not to win" , "Cast, lob" , "Hurl, fling" .)