Head off duck trapping seabird — it’s never-ending! (7)
I believe the answer is:
eternal
'never-ending' is the definition.
(both can mean lasting forever)
'head off duck trapping seabird' is the wordplay.
'head off duck' becomes 'eal' (I can't justify this - if you can you should give a lot more credence to this answer).
'trapping' means one lot of letters goes inside another.
'seabird' becomes 'tern' (tern is a kind of seabird).
'eal' placed around 'tern' is 'ETERNAL'.
'it's' is the link.
(Other definitions for eternal that I've seen before include "Infinite - endless" , "Never-ending, existing always" , "Undying" , "unchanged by time" , "Timeless" .)