Rodent in middle of the stream, swimming (7)
I believe the answer is:
hamster
'rodent' is the definition.
(hamster is a kind of rodent)
'middle of the stream swimming' is the wordplay.
'middle of' means to look at the middle letters.
'swimming' is an anagram indicator.
The central letter of 'the' is 'h'.
'stream' with letters rearranged gives 'amster'.
'h'+'amster'='HAMSTER'
'in' acts as a link.
(Other definitions for hamster that I've seen before include "Pet rodent, may be golden" , "Furry pet" , "The arms (anag)" , "Little beast" , "Small pet rodent with pouched cheeks" .)