A second seabird on the back of a liner, say (6)
I believe the answer is:
astern
'on the back of a liner say' is the definition.
(astern means at the back of a boat)
'a second seabird' is the wordplay.
'second' becomes 's' (common abbreviation).
'seabird' becomes 'tern' (tern is a kind of seabird).
'a'+'s'+'tern'='ASTERN'
(Other definitions for astern that I've seen before include "Abaft" , "back for sailors" , "Behind (naut.)" , "Behind (a boat)" , "Behind at sea" .)