Is glyphosate safe to spray in my standing crop?

No, do not spray glyphosate on a standing crop like wheat. Glyphosate is a non-selective, broad-spectrum herbicide, which means it kills almost any green plant it touches, including your crop, so spraying it over standing wheat would damage or kill the wheat along with the weeds. Glyphosate is meant for non-crop uses: clearing weeds before sowing (a pre-sowing or stale-seedbed knockdown), clearing field bunds, channels, orchards (as a directed spray away from trunks) and fallow or waste land, where no crop is present or where it can be kept off the crop. For weeds inside a standing wheat crop you must use selective herbicides registered for wheat, such as the grass and broadleaf products meant for that crop, which kill the weeds without harming the wheat. The shopkeeper's advice here is unsafe. Use only herbicides recommended for your specific crop and situation, follow the label exactly, and confirm with your KVK before spraying anything new, because a non-selective herbicide in a standing crop can destroy it.