More than one bird races to catch island rodents (7)
I believe the answer is:
titmice
'more than one bird' is the definition.
(titmouse is a kind of bird)
'races to catch island rodents' is the wordplay.
'races' becomes 'tt' (Isle of Man Tourist Trophy races).
'to catch' means one lot of letters goes inside another.
'island' becomes 'i'.
'rodents' becomes 'mice' (mouse is a kind of rodent).
'tt' going around 'i' is 'tit'.
'tit'+'mice'='TITMICE'
(Other definitions for titmice that I've seen before include "Small birds" , "Small, active songbirds" , "Little singers" , "little birds" .)