Neanderthal starting on all fours (3)
I believe the answer is:
oaf
'neanderthal' is the definition.
(I've seen this in another clue)
'starting on all fours' is the wordplay.
'starting' says to take the initial letters.
The initial letters of 'on all fours' is 'OAF'.
(Other definitions for oaf that I've seen before include "Lout; dolt" , "Goon" , "An ill-mannered lout" , "Stupid lout" , "Lout, lummox" .)