I heartlessly try following European bird for an extremely long time (8)
I believe the answer is:
eternity
'an extremely long time' is the definition.
(I've seen this before)
'i heartlessly try following european bird' is the wordplay.
'heartlessly' suggests removing the centre.
'following' means one lot of letters go next to another.
'european' becomes 'E' (abbreviation e.g. EU).
'bird' becomes 'tern' (tern is a kind of bird).
'try' with its middle taken out is 'ty'.
'i'+'ty'='ity'
'e'+'tern'='etern'
'ity' put after 'etern' is 'ETERNITY'.
'for' acts as a link.
(Other definitions for eternity that I've seen before include "the poet palmed in an hour?" , "Infinity" , "Time after death" , "That's a very long time" , "Perpetuity" .)