Complex sentence: Defectors are "not" considered agents as agents are in place to function for an intelligence service and defectors are not, however some consider that defectors in place are agents as long as they have not yet defected.
Split into simpler sentences: Defectors are "not" considered agents as agents are "in place" to function for an intelligence service and defectors are not. However, some consider that defectors "in place" are agents as long as they have not yet defected.