Efficient secretary blocking message (7)
I believe the answer is:
capable
'efficient' is the definition.
(I've seen this before)
'secretary blocking message' is the wordplay.
'secretary' becomes 'PA' (abbreviation for personal assistant).
'blocking' indicates putting letters inside (to block can mean to plug a gap in something).
'message' becomes 'cable' (as in sending a cable to someone).
'pa' going within 'cable' is 'CAPABLE'.
(Other definitions for capable that I've seen before include "Competent, having a certain ability" , "qualified" , "'Clever, able (7)'" , "accomplished" , "Able, proficient" .)