It's terribly rude to side-track (6)
I believe the answer is:
detour
'track' is the definition.
(I know that detour is a type of track)
'it's terribly rude to' is the wordplay.
'it's terribly' indicates anagramming the letters.
'rude'+'to'='rudeto'
'rudeto' with letters rearranged gives 'DETOUR'.
'side' acts as a link.
This may not be correct. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for detour that I've seen before include "Deviation from direct course" , "Circuitous alternative to direct path" , "Deviation from the usual route" , "Diversion" , "A way round, say, road works" .)