Is it OK to have a single front chainring paired with 7 speed at the back? I'm planning to have a single front chain ring to be paired with my 7 speed at the back. I'm still saving up for a 10 speed.
Without taking into account whether it's a good idea or not; yes it's possible, with a few caveats.

You have two issues:

Chain line i.e., alignment between the front rings and rear cassette.

The single ring needs to be aligned with the center of the cassette so you minimize the lateral deflection on the chain and avoid excessive wear on the chain and sprockets.

With a 7 speed rear its likely that you have a triple set of rings. If so just us the middle position for the single front ring as it's aligned with the middle of the cassette. If you have a double, things are trickier but you may be able mount the ring on the inboard side of the spider.

Dropping the chain

The front derailleur helps to keep the chain on the front rings. Outer rings help keep the chain on inner rings. With an unaided single front ring there is a greater chance of dropping the chain from it.

1x11 setups feature a narrow-wide chainring that have alternating narrow and wide teeth that fit between the inner and outer chain connecting plates to more firmly hold on to the chain. If you are looking to do this conversion as cheaply as possible you probably don't want to get a new chainring. Leave the front derailleur in place to act as a chainguide.

You don't say whether you are saving for a whole 1x11 bike, or or planning to replace the drivetrain on you current bike. In either case this article may be of use to you. If the latter, bear in mind you will need to check compatibility between your freehub body and an 11 speed cassette.