One not yet out? European leaves Cornish town (3)
I believe the answer is:
bud
'one not yet out?' is the definition.
(part of a plant not yet out)
'european leaves cornish town' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'leaves' indicates named letters should be taken away.
'cornish town' becomes 'Bude' (Bude is a town in Cornwall).
'bude' with 'e' removed is 'BUD'.
(Other definitions for bud that I've seen before include "Small knob that will develop into a leaf or flower" , "Undeveloped flower" , "It will eventually become a leaf or flower" , "Shoot, sprout" , "Unopened flower" .)