Throwing a six on the line (4)
I believe the answer is:
axis
'the line' is the definition.
(axis is a kind of line)
'throwing a six on' is the wordplay.
'throwing' is an anagram indicator (letters get thrown around).
'on' means one lot of letters go next to another.
'a'+'six'='asix'
'asix' with letters rearranged gives 'AXIS'.
(Other definitions for axis that I've seen before include "Imaginary line around which object rotates" , "World War two Italy-Germany alliance" , "from Germany and Italy, originally" , "Centre about which something turns" , "line to be turned round" .)