Some exotic hummingbirds mate (4)
I believe the answer is:
chum
'mate' is the definition.
(I've seen this before)
'some exotic hummingbirds' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'CHUM' is hidden within 'exotic hummingbirds'.
(Other definitions for chum that I've seen before include "Pal, mate" , "Informally, a close friend" , "Salmon" , "Close friend (colloq.)" , "Pal, buddy" .)