Dismiss half of costs on line (5)
I believe the answer is:
expel
'dismiss' is the definition.
(I've seen this in another clue)
'half of costs on line' is the wordplay.
'half of' suggests halving the letters.
'costs' becomes 'expenses'.
'on' means one lot of letters go next to another.
'line' becomes 'l' (used when specifying particular lines from a poem).
'expenses' cut in half is 'expe'.
'expe'+'l'='EXPEL'
(Other definitions for expel that I've seen before include "Drive out - dispossess" , "Ban from school" , "Force to leave" , "Reject" , "Discharge in disgrace" .)