Secretary to reveal a bit of leg (7)
I believe the answer is:
patella
'bit of leg' is the definition.
('patella' is part of 'leg')
'secretary to reveal a' is the wordplay.
'secretary' becomes 'PA' (abbreviation for personal assistant).
'to reveal' becomes 'tell' (I've seen this before).
'pa'+'tell'+'a'='PATELLA'
(Other definitions for patella that I've seen before include "The formal kneecap" , "showing a bit of leg" , "bit of body usually under trousers!" , "Joint cover" , "Knee bone" .)