Pupils once mauling tailless bats (6)
I believe the answer is:
alumni
'pupils' is the definition.
(I have seen 'Former pupils' mean 'alumni' so perhaps 'pupils' could also mean 'alumni')
'mauling tailless bats' is the wordplay.
'tailless' means to remove the last letter (the tail of the word is removed).
'bats' is an anagram indicator (slang for mad).
'mauling' with its last letter taken off is 'maulin'.
'maulin' with letters rearranged gives 'ALUMNI'.
'once' is the link.
(Other definitions for alumni that I've seen before include "Past pupils of the old school" , "Put too much strain on, as Revenue Commissioners do" , "Old boys and girls" , "Past pupils or graduates" , "Graduates of an institution" .)