One in Singapore crooned (4)
I believe the answer is:
sang
'crooned' is the definition.
'one in singapore' is the wordplay.
'one' becomes 'an' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'singapore' becomes 'sg' (I can't explain this - if you can you should give a lot more credence to this answer).
'an' put inside 'sg' is 'SANG'.
Can you help me to learn more?
(Other definitions for sang that I've seen before include "Spilt the beans" , "squealed" , "Chanted" , "Took part in a vocal performance" , "Did numbers" .)