An animal beginning to swim perfectly (5)
I believe the answer is:
stoat
'an animal' is the definition.
(type of rodent)
'beginning to swim perfectly' is the wordplay.
'beginning to' indicates taking the first letters.
'perfectly' becomes 'to a t' ('to a T' or 'to a tee' can mean perfectly).
The first letter of 'swim' is 's'.
's'+'toat'='STOAT'
(Other definitions for stoat that I've seen before include "European ermine" , "Weasel-like mammal" , "Provider of ermine" , "Short-tail weasel" , "Source of ermine" .)