Yahoo, say, backed up by secretary's young assistant (7)
I believe the answer is:
pageboy
'young assistant' is the definition.
'yahoo say backed up by secretary's' is the wordplay.
'yahoo' becomes 'yob' (both can mean an unruly person).
'say' becomes 'eg'.
'backed up' is a reversal indicator.
'by' says to put letters next to each other.
'secretary' becomes 'pa' (I've seen this in other clues).
'yob'+'eg'='yobeg'
'yobeg' in reverse letter order is 'geboy'.
'geboy' put after 'pa' is 'PAGEBOY'.
(Other definitions for pageboy that I've seen before include "Bride's attendant" , "Young bridal attendant" , "Youthful hotel attendant" , "Hairstyle - child at wedding" , "Hotel worker; hairstyle" .)