Offer to keep training two-footed animal (5)
I believe the answer is:
biped
'two-footed animal' is the definition.
(I've seen this before)
'offer to keep training' is the wordplay.
'offer' becomes 'bid' (bid is a kind of offer).
'to keep' indicates putting letters inside.
'training' becomes 'PE' (Physical Education).
'bid' placed around 'pe' is 'BIPED'.
(Other definitions for biped that I've seen before include "Walker on two feet" , "One walking on two legs" , "Using two pins" , "Girl, say" , "One standing on two feet" .)