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' is an insertion indicator.
'empty' means to remove the middle letters.
'locker' with its centre taken out is 'lr'.
'emu' going inside '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" .)