Asakusa for a slightly cramped but very buzzy Japanese izakaya, just behind Mornington Crescent station, about 15 min walk to Camden tube. You'll have to call to book a table for dinner, though - they always turn away walk-ins. I would recomend their cooked food rather than sushi/sashimi as they only have one chef and can be slow, plus it isn't their strong point anyway. The grilled chicken hearts are to die for!
I swear this exact same question got asked, and thoroughly answered, a couple of years ago, but I can't find it in the search results. Does Chowhound delete threads that are over a 18 months old? What a shame to lose the older threads.
Belgo Noord, across from the Roundhouse isn't too bad, if you're happy with a bucket of mussels and some chips. Yum Cha, further down Chalk Farm Road is good. There's some options on Regents Park Road, just over the bridge from Chalk Farm: Lemonia and Lemani for Greek food, and Trojka for Russian/eastern European. Also, you might consider one of the excellent pizzas at the Lansdowne Pub on Gloucester Ave. in Primrose Hill. El Parador, down by Mornington Crescent station is nice for tapas. It's been over a year since I lived in the area, so not sure if there are new places to add.
This thread might also be helpful:
Easy - Made in Camden in the Roundhouse itself is fantastic - really nice creative small plate dishes - far better than you would expect for a venue restaurant.
Can't recommend highly enough - http://www.madeincamden.com/MiCv2/con...
If you don't fancy that, Market on Parkway (10/15 min walk) is also very good for modern British.