Energy-guzzling vehicle by softly undulating Welsh town (10)
I believe the answer is:
caerphilly
'welsh town' is the definition.
(I've seen this before)
'energy-guzzling vehicle by softly' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mch).
'guzzling' means one lot of letters goes inside another.
'vehicle' becomes 'car' (car is a kind of vehicle).
'by' means one lot of letters go next to another.
'softly' becomes 'philly' (I am not sure about this - if you are sure you should believe this answer much more).
'e' placed into 'car' is 'caer'.
'caer'+'philly'='CAERPHILLY'
'undulating' is the link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for caerphilly that I've seen before include "Mild white cheese" , "Town north of Cardiff" , "Welsh town" .)