Parent of son expelled by teacher (5)
I believe the answer is:
mater
'parent' is the definition.
(I know that mater is a type of mother)
'son expelled by teacher' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'expelled by' suggests deleting specific letters.
'teacher' becomes 'master' (I've seen this before).
'master' with 's' taken out is 'MATER'.
'of' acts as a link.
(Other definitions for mater that I've seen before include "One marrying" , "Mum" , "having children" , "The alma . . . . . Is the old school (5)" , "Mother (sl.)" .)