Girl causes a number to leave the teaching group (4)
I believe the answer is:
lass
'girl' is the definition.
(lass can informally mean a girl)
'number to leave the teaching group' is the wordplay.
'number' becomes 'C' (C is an example).
'to leave' suggests deleting specific letters.
'the teaching group' becomes 'class' (I've seen this before).
'class' with 'c' taken away is 'LASS'.
'causes a' is the link.
(Other definitions for lass that I've seen before include "Scottish girl" , "Sweet resident of Richmond Hill?" , "Girl (of Richmond Hill?)" , "Sweetheart" , "Familiar form of address for female" .)