River offshoot

The answer to the crossword question “River offshoot is ARM” is likely to be a four-letter word starting with the letter A. In geography, an arm is a narrow channel of a river or a sea that branches off from the main body of water. Examples include the Gulf of Guinea’s Bight of Benin and Hudson Bay’s James Bay. In military terminology, ARM may refer to Armored or Armament. It may also be a reference to a human limb, which is an anatomical term. However, considering the context of the clue provided, it is more likely to be the first definition, referring to a river offshoot.