The phrase "a decent fist" is correct and usable in written English.
It can be used in contexts where you are describing a reasonable or satisfactory level of effort or performance, often in a metaphorical sense.
Example: "He made a decent fist of the project, managing to meet most of the deadlines despite the challenges."
Alternatives: "a good attempt" or "a fair effort".
Theresa May might make a decent fist of that.
We make a decent fist of it, and Jacques seems pleased with our enthusiasm.
The state can make a decent fist of bringing services to them, but it cannot bring jobs.
Yet although the banks dismissed the report's "flawed analysis", it seems to have made a decent fist of it.
He had been at the centre and he may even have made a decent fist of things.
The kind of guy, in other words, you would quite like to make a decent fist of it.
Having established an intriguing multiverse for Lin Shaye's homebody psychic to investigate in 2013's second instalment, this prequel makes an entirely decent fist of a story about a bedbound teenager (the bright, sympathetic Stefanie Scott) attracting demonic interest after reaching out to her late mother.
Before Mike Patton arrived, vocalist Chuck Mosley made a decent enough fist of frontman duties, despite mysteriously getting struck down with a cold whenever the band had studio time.
He was clearly more comfortable with the former, but he made a decent enough fist of maintaining the tension during scenes that make next to no sense.
Winner: Dawkins Mensch makes a decent enough fist of trolling – starting fights with Alastair Campbell here, claiming that depressed people should go jogging there – but she's a gnat in the face of the towering cave troll that is her opponent.
Carolyn, Suzie, Tisha and Tanya made a decent enough fist of it with their 'Dumb' single, but the hip-hop-flavoured 'On My Knees' (also a hit) remains an embarrassment, its cloddish rhymes sounding like fingernails down a chalkboard, rather than the All Saints's 'Never Ever'.
