Secretaries tease one persistently snappy crowd (9)
I believe the answer is:
paparazzi
'persistently snappy crowd' is the definition.
(press photographers who snap photos)
'secretaries tease one' is the wordplay.
'secretaries' becomes 'papa' (I can't explain this - if you can you should give a lot more credence to this answer).
'tease' becomes 'razz' (to razz someone is to tease them).
'one' becomes 'I' (Roman numeral).
'papa'+'razz'+'i'='PAPARAZZI'
(Other definitions for paparazzi that I've seen before include "Photographers who pursue celebrities" , "Press photographers" , "Freelance photographers, from Italy" , "Freelance snappers" , "Italian freelance photographers, very intrusive" .)