The Collaborative International Dictionary
part-timer
part-timer \part-timer\ n. Somone who works part-time; a part-time employee. Contrasted with full-timer.
Wiktionary
part-timer
n. 1 someone who is employed, or does something, part-time 2 (context cricket English) a part-time bowler
WordNet
part-timer
n. someone who works less than the customary or standard time
Usage examples of "part-timer".
Andrea had taken over the executive staff duties, and Charlie the environmental research, though he too was a part-timer, and telecommuting much of the time.
Part-timers are hired to fill out the shifts, though many are not trained as emergency medical technicians, as the full-time lifeguards usually are.
Now we've got eight full-time employees, plus those part-timers Frank Mettler works with.