Supply lines (6)
I believe the answer is:
prompt
'supply lines' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for prompt that I've seen before include "Supply a forgotten line to an actor" , "Spurs do this" , "Bring about; done without delay" , "Punctual; instigate" , "Speedy - help for forgetful actor" .)