It's hard to tell what the exact problem is here without seeing how this has been constructed, but here are some ways you could go about creating it anyway.
Method 1: This will cut up everything into individual closed paths.
- Make sure your design is converted to paths using Path > Object to Path - this will bake in any path effects. Ungroup everything a couple of times to make sure there are no groups. Select it all and make it a combined path.
Everything shown here is one combined path, including the pentagon
Draw a rectangle to surround the entire design, and set a fill so you can see what is going on, and move this rectangle to the bottom of the stack using End
Select everything, and do Path > Division
Select and delete the pieces outside the pentagon
An example
Method 2: Using a clipping path. I will assume the design is one group. There's no need to combine paths using this method, a group will do. Also no need to bake in any path effects. Using this method nothing is actually deleted, merely hidden by the clip.
Draw your pentagon over the design group, and then Ctrl+C Copy the pentagon to the clip board.
Select the design, and the pentagon, and do Object > Clip > Set Clip
Do Edit > Paste in Place to get the pentagon back
An example
Method 3: Use this you just want to cut up all the paths without making closed shapes.
Select the design and do Path > Object to Path to bake in any path effects, ungroup several times, and Combine the design as one path
Draw the pentagon on top, and copy the pentagon to the clip board
Select all and do Path > Cut Path
Do Edit > Paste in Place to bring back the pentagon
Select all the pieces outside the pentagon and delete
