Young fliers terribly slow to welcome jets without wings (6)
I believe the answer is:
owlets
'young fliers' is the definition.
(young owls)
'terribly slow to welcome jets without wings' is the wordplay.
'terribly' indicates anagramming the letters.
'to welcome' is an insertion indicator (in sense of 'inviting in').
'without wings' says to take the centre (the letters on the edges might be called wings).
The middle of 'jets' is 'et'.
'slow' with letters rearranged gives 'owls'.
'owls' going around 'et' is 'OWLETS'.
(Other definitions for owlets that I've seen before include "Young wise birds" , "Baby wise old birds" .)