Drop round line for rod (4)
I believe the answer is:
axle
'rod' is the definition.
(axle is a kind of rod)
'drop round line' is the wordplay.
'drop' becomes 'axe' (I've seen this in another clue).
'round' indicates putting letters inside.
'line' becomes 'l' (used when specifying particular lines from a poem).
'axe' enclosing 'l' is 'AXLE'.
'for' acts as a link.
(Other definitions for axle that I've seen before include "Metal pin on which wheel revolves" , "Rotating spindle" , "revolver in one" , "Spindle between wheels" , "need of wheeled cart?" .)