A quarrel involving thrower's last dart (5)
I believe the answer is:
arrow
'dart' is the definition.
(I've seen this before)
'a quarrel involving thrower's last' is the wordplay.
'quarrel' becomes 'row' (synonyms).
'involving' means one lot of letters goes inside another.
'last' indicates one should take the final letters.
The last letter of 'thrower' is 'r'.
'row' placed around 'r' is 'rrow'.
'a'+'rrow'='ARROW'
(Other definitions for arrow that I've seen before include "broadly distinguishing old convict" , "Missile with a straight thin shaft" , "one found near Bow?" , "It may indicate" , "Narrow missile" .)