Efficient message covering a page (7)
I believe the answer is:
capable
'efficient' is the definition.
(I've seen this before)
'message covering a page' is the wordplay.
'message' becomes 'cable' (cable is a kind of message).
'covering' is an insertion indicator.
'page' becomes 'p'.
'a'+'p'='ap'
'cable' going around 'ap' is 'CAPABLE'.
(Other definitions for capable that I've seen before include "Of a person, competent" , "Competent, having a certain ability" , "Adept" , "Efficient" , "accomplished" .)