Lout starts off on all fours (3)
I believe the answer is:
oaf
'lout' is the definition.
('oaf' can be a synonym of 'lout')
'starts off on all fours' is the wordplay.
'starts off' suggests taking the first letters.
The initial letters of 'on all fours' is 'OAF'.
(Other definitions for oaf that I've seen before include "Awkward stupid person" , "Bumpkin" , "Stupid or boorish man" , "Boor or lout" , "Oik" .)