Aircraft's self-assembly set with ultimate in glue (4)
I believe the answer is:
kite
'aircraft's' is the definition.
(I've seen this before)
'self-assembly set with ultimate in glue' is the wordplay.
'self-assembly set' becomes 'kit'.
'with' says to put letters next to each other.
'ultimate in' indicates one should take the final letters.
The last letter of 'glue' is 'e'.
'kit'+'e'='KITE'
(Other definitions for kite that I've seen before include "It flies with string attached" , "Toy flown at the end of a string" , "Raptor - flying toy" , "plane" , "Toy that flies on end of string" .)