First, turn off the power to the light. Then check the socket for tightness and the connection behind the trim plate. If the flickering persists, chase the wire back to the 1st junction box (easier said than done sometimes) and check to see the wire nuts are the proper size, in good shape and tight (to one full twist of the wires). There is possibly a poor outlet to which the power is connected that could be causing the issue and be needing replacement. Be sure to use your wire tester before sticking your fingers into things.
John
There could be several issues here. The most likely is a loose wire termination connection at either a junction box, switch or at the fixture. Other causes could be a faulty switch or damaged conductor.