Little swimmers bit people from Warsaw, say (8)
I believe the answer is:
tadpoles
'little swimmers' is the definition.
(I've seen this before)
'bit people from warsaw say' is the wordplay.
'bit' becomes 'tad' (both can mean a small amount).
'people from warsaw say' becomes 'poles' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'tad'+'poles'='TADPOLES'
Can you help me to learn more?
(Other definitions for tadpoles that I've seen before include "Baby frogs" , "Young frogs" , "Pond-life" , "Larvae of frogs or toads" , "Little swimmers" .)