Self-assurance of reflective secretary shot up, minutes in (6)
I believe the answer is:
aplomb
'self-assurance' is the definition.
(I've seen this before)
'reflective secretary shot up minutes in' is the wordplay.
'reflective' says the letters should be written backwards.
'secretary' becomes 'pa' (abbreviation for personal assistant).
'shot up' becomes 'lob' (I've seen this before).
'minutes' becomes 'm' (common abbreviation).
'in' indicates putting letters inside.
'pa' in reverse letter order is 'ap'.
'lob' enclosing 'm' is 'lomb'.
'ap'+'lomb'='APLOMB'
'of' is the link.
(Other definitions for aplomb that I've seen before include "Cool self-assurance" , "Self-satisfied assurance" , "Pompous dignity or poise" , "'Poise, sang-froid (6)'" , "Calm self-assurance and poise" .)