Radius, not quite so long, is set? (5)
I believe the answer is:
radio
'is set?' is the definition.
(I've seen this before)
'radius not quite so long' is the wordplay.
'radius' becomes 'r' (abbreviation in geometry).
'not quite' means to remove the last letter.
'so long' becomes 'adios' ('adios' can be a synonym of 'so long').
'adios' with its last letter removed is 'adio'.
'r'+'adio'='RADIO'
(Other definitions for radio that I've seen before include "Sound receiver" , "Transmit" , "Communications device" , "Electronic medium of communication" , "Marconi invented it" .)