Go on all fours (5)
I believe the answer is:
crawl
'go on all fours' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for crawl that I've seen before include "Unusually slow rate of movement" , "Grovel" , "Baby-like advance" , "Proceed on hands and knees" , "Slow advance" .)