Thrifty group sharing vehicle to save energy (7)
I believe the answer is:
careful
'thrifty' is the definition.
(synonyms)
'group sharing vehicle to save energy' is the wordplay.
I cannot quite understand how this works, but
'vehicle' could be 'car' (car is a kind of vehicle) and 'car' is present in the answer.
'energy' could be 'e' (physics symbol as in E=mc2) and 'e' is present in the answer.
The remaining letters 'ful' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for careful that I've seen before include "Exercising caution" , "Painstaking" , "Thrifty" , "Thorough" , "'Prudent, cautious (7)'" .)