Bird will resist eating nuts, initially (4)
I believe the answer is:
wren
'bird' is the definition.
(I know that wren is a type of passeriform bird)
'will resist eating nuts initially' is the wordplay.
'initially' says to take the initial letters.
The first letters of 'will resist eating nuts' is 'WREN'.
(Other definitions for wren that I've seen before include "Once a woman in the Royal Navy" , "Very small brown songbird (4)" , "A small garden bird" , "Bird - architect, 1632-1723" , "seafarer" .)