Eponymous tortoise beginning to gain, 400m back? (8)

'eponymous tortoise' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'beginning to gain 400m back?' is the wordplay.
'beginning to' indicates taking the first letters.
'p00m' becomes 'alapa' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'back?' becomes 'go' (backing is a kind of going).
The first letter of 'gain' is 'g'.
'g'+'alapa'+'go'='GALAPAGO'

Can you help me to learn more?