"At head" typically means "in charge" or "leading," so antonyms for this phrase might include "subordinate," "follower," or "assistant." Other antonyms could include "behind the scenes," "supporting role," or "underling." Depending on the context, other antonyms could be used to convey the opposite meaning of being in charge or leading, such as "incompetent," "ineffective," or "irresponsible." Ultimately, the most effective antonym will depend on the specific situation and the tone desired. Regardless of the antonym used, it should convey the opposite meaning of "at head.