"Maintain runway heading"
Seems simple enough to me!
Dr
PS:
ATC -
"Doc, turn left, heading 270".
Dr -
"Ah, you want me to head 270 or track 270" ?
ATC -
"Geez Doc, its not rocket science. When I want you to take up a heading, I say "heading", and if I want you to track, I say "track"!