Increase twofold (6)
I believe the answer is:
double
'increase twofold' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for double that I've seen before include "Designed for two users" , "Increase twofold" , "Offence of leaving one's car where it blocks another" , "Bed for two people" , "Twice, the size say" .)