In thoughtful mood after European vote becomes costly (9)
I believe the answer is:
expensive
'costly' is the definition.
(similar in meaning)
'in thoughtful mood after european vote' is the wordplay.
I cannot quite see how this works, but
'thoughtful' could be 'pensive' (similar in meaning) and 'pensive' is present in the answer.
'european' could be 'e' (abbreviation e.g. EU) and 'e' is present in the answer.
'vote' could be 'x' (how votes are marked) and 'x' is located in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'becomes' acts as a link.
Can you help me to learn more?
(Other definitions for expensive that I've seen before include "Charging a lot" , "not going for a song" , "priceless" , "Very costly or dear" , "Highly-priced" .)