Multiply by three (6)
I believe the answer is:
triple
'multiply by three' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for triple that I've seen before include "Threefold, treble" , "make much longer" , "Increase threefold" , "Consisting of three" , "Let rip with a threesome" .)