Second bird's tail (5)
I believe the answer is:
stern
'tail' is the definition.
(back of a ship or boat)
'second bird's' is the wordplay.
'second' becomes 's' (common abbreviation).
'bird' becomes 'tern' (tern is a kind of bird).
's'+'tern'='STERN'
(Other definitions for stern that I've seen before include "Grimly serious; back" , "Grave; rear" , "Strict, grimly serious" , "at the back of the galley!" , "Rents out at rear of boat" .)