Endless delay regularly besetting seabird by lake (7)
I believe the answer is:
eternal
'endless' is the definition.
(thesaurus)
'delay regularly besetting seabird by lake' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'besetting' is an insertion indicator.
'seabird' becomes 'tern' (I've seen this before).
'by' means one lot of letters go next to another.
'lake' becomes 'l'.
The alternate letters of 'delay' are 'ea'.
'ea' enclosing 'tern' is 'eterna'.
'eterna'+'l'='ETERNAL'
(Other definitions for eternal that I've seen before include "Permanent, without end" , "going on for ever" , "Constant" , "Valid for all time" , "Without beginning or end" .)