Swam back around a hundred birds (6)
I believe the answer is:
macaws
'birds' is the definition.
(macaw is a kind of bird)
'swam back around a hundred' is the wordplay.
'back' is a reversal indicator.
'around' is an insertion indicator.
'hundred' becomes 'c' (Roman numeral).
'swam' back-to-front is 'maws'.
'a'+'c'='ac'
'maws' going around 'ac' is 'MACAWS'.
(Other definitions for macaws that I've seen before include "More than one bird" , "Brightly-coloured parrots" , "Long-tailed parrots of Central and South America" , "Birds" , "Long-tailed, brilliantly-coloured parrots" .)