Lengthens (7)
I believe the answer is:
extends
'lengthens' is the definition.
(extending is a kind of lengthening)
This is all the clue.
(Other definitions for extends that I've seen before include "Reaches or stretches out, say the hand of friendship" , "Makes longer" , "Lengthens" , "Stretches out over a distance" , "Enlarges" .)