Animal or bird entering empty locker (5)
I believe the answer is:
lemur
'animal' is the definition.
(lemur is a kind of animal)
'bird entering empty locker' is the wordplay.
'bird' becomes 'emu' (emu is a type of bird).
'entering' indicates putting letters inside.
'empty' says to hollow out the word (remove centre letters).
'locker' with its centre taken out is 'lr'.
'emu' placed within 'lr' is 'LEMUR'.
'or' is the link.
(Other definitions for lemur that I've seen before include "one in the bush?" , "Tree-living primate" , "Monkey-like animal of Madagascar" , "Bushy-tailed primate" , "Arboreal mammal common to Madagascar" .)