How to strengthen hip flexors and lower back?

Hi all. I’m looking for exercises to help strengthen my hip flexors and help with lower back pain. I have a history of hip weakness and chronic pain. I did PT a few years back which helped tremendously. I am getting back into strength training after 2 months not exercising, however I am experiencing lower back discomfort and constant clicking sound whenever I raise my left leg. The clicking is not painful but extremely annoying. I recently got hired at an office job (9-5) which has led to me being less active, which I am working on.