Former pupil sent icy order as an expletive (9)
I believe the answer is:
obscenity
'an expletive' is the definition.
(thesaurus)
'former pupil sent icy order' is the wordplay.
'former pupil' becomes 'OB' (abbreviation for 'Old Boy' - private school pupil).
'order' indicates anagramming the letters.
'sent'+'icy'='senticy'
'senticy' with letters rearranged gives 'scenity'.
'ob'+'scenity'='OBSCENITY'
'as' is the link.
(Other definitions for obscenity that I've seen before include "Strong language" , "Smut" , "indecency" , "Bad language" , "pornography?" .)