One directs roofer to add extra line (6)
I believe the answer is:
tiller
'one directs' is the definition.
(someone directing a ship)
'roofer to add extra line' is the wordplay.
I cannot really see how this works, but
'to' could be 'til' (I've seen this in another clue) and 'til' is located in the answer.
'line' could be 'l' (used when specifying particular lines from a poem) and 'l' is present in the answer.
The remaining letters 'er' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for tiller that I've seen before include "Boat's steering arm" , "One who works the land" , "Horizontal bar to turn rudder" , "steering contraption" , "What boatman uses" .)