I have just enough miles to do Syd-Barcelona return in economy.Is it possible to pay some sectors and use Krisflyer miles for Business Class say SIN-BCN-SIN only?
If you mean paying cash to Singapore and then use points to Barcelona, then yes, but you’d have to make separate bookings.
The answer is yes, as @djtech has said – you just need to make two separate bookings. However, if you’ve got “just enough” miles to do Sydney to Barcelona in economy (53000 KrisFlyer miles each way) that means you have just over 106000 KrisFlyer miles.
Just be aware that a one-way Business redemption from Sydney to Singapore is 62000 miles, and from Singapore to anywhere in Europe is 92000 miles (the KrisFlyer redemption table is here – scroll down to the “Saver” awards rather than the more expensive “Advantage” awards). That means that you’ve only got enough points at the moment for one, one-way flight in Business. You might like to consider either flying Premium Economy, or taking a little more time to accumulate more points.
All the best!