How to improve application release management when having 3 environments

Hey, guys! In our small company application, both frontend and backend side, are released by having three environments - development, staging and production. The biggest problem that we prepare feature A in backend let's say and we merge that code to development, but frontend also asks later for mocks to work with feature B. We also deploy it to development. Now our client says that feature A is need in production env ASAP if it's finished and working - here we have a problem where we can't merge development environment, because maybe feature A is finished, but feature B with mocks can't go to production or staging. How you would suggest to solve this problem - and how we should restructure our Application release management to improve it. Thank you