A number swam back to protect the birds (6)
I believe the answer is:
macaws
'birds' is the definition.
(macaw is a kind of bird)
'a number swam back to protect' is the wordplay.
'number' becomes 'c' (C is an example).
'back' shows that the letters should be reversed in order.
'to protect' means one lot of letters goes inside another (inserted letters are held or protected).
'swam' reversed gives 'maws'.
'a'+'c'='ac'
'ac' inserted into 'maws' is 'MACAWS'.
'the' is the link.
(Other definitions for macaws that I've seen before include "Brightly-coloured parrots" , "Birds" , "Long-tailed parrots of Central and South America" , "More than one bird" , "Long-tailed, brilliantly-coloured parrots" .)