on defense

:  the act or action of defending (see defend)

:  the denial, answer, or plea (see plea 2b) of one against whom a criminal or civil action is brought :  a defendant 's denial, answer, or plea

:  capability of resisting attack

:  to take specific defensive action against (an opposing team or player or an offensive play) :  to attempt to keep (an opponent or play) from scoring in a game or contest