Boarding jet, one flier — go on! (4,2,3)
I believe the answer is:
spit it out
'go on' is the definition.
(phrase used to tell someone to carry on talking)
'boarding jet one flier' is the wordplay.
'boarding' is an insertion indicator.
'jet' becomes 'spout' (I have seen 'Jet of liquid' mean 'spout' so perhaps 'jet' could also mean 'spout').
'one' becomes 'i' (Roman numeral).
'flier' becomes 'tit' (type of bird).
'i'+'tit'='itit'
'spout' going around 'itit' is 'SPIT IT OUT'.
(Other definitions for spit it out that I've seen before include "Tell me forthwith!"" , "Come on, say what's on your mind - expectorate if you wish" , "Come on, say it quickly - or expectorate" , "Go on!" , "Come on, say what you think - expectorate" .)