Leader of ecologists regularly ringed eagle (4)
I believe the answer is:
erne
'eagle' is the definition.
(erne is a kind of eagle)
'leader of ecologists regularly ringed' is the wordplay.
'leader of' indicates taking the first letters.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The initial letter of 'ecologists' is 'e'.
The alternate letters of 'ringed' are 'rne'.
'e'+'rne'='ERNE'
(Other definitions for erne that I've seen before include "Fish-eating sea eagle" , "Lough of the North" , "'Sea-eagle, river and lough (4)'" , "Large bird" , "Flier" .)