Can I secure a pantry cleat where there’s no stud?

For pantry storage, I’d avoid relying on glue against drywall. If you can take the cleat off, replace it with a longer one that reaches a stud and screw it into the stud. That gives the shelf a much more dependable support than asking the drywall to carry the load.

You can usually drill through the cleat and fit a toggle anchor, provided the anchor suits the wall cavity and thickness. Use a washer under the screw head so it bears against the wood instead of slipping into the oversized hole. Follow the anchor’s installation directions, and don’t treat its advertised rating as a guarantee for a loaded pantry shelf. The load, wall condition, and shelf leverage all matter.

Another option is to support that end from below with a vertical piece running down to the floor, then support the shelves above it as needed. That shifts much of the weight to the floor instead of the drywall. Make sure the upright sits firmly and account for any baseboard so it doesn’t lean or rock.