Bag a pigeon (7)
I believe the answer is:
carrier
I believe this clue is a double definition.
'bag' is the first definition.
(I've seen this before)
'a pigeon' is the second definition.
(carrier pigeon is a kind of pigeon)
(Other definitions for carrier that I've seen before include "Aircraft- - -" , "Conveyor of goods or transmitter of disease" , "Ark Royal?" , "Large naval ship" , "Person able to transmit a disease, but lacking its symptoms" .)