A quiet telegram about being efficient (7)
I believe the answer is:
capable
'efficient' is the definition.
(I've seen this before)
'a quiet telegram about' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'telegram' becomes 'cable' (cable is a kind of telegram).
'about' is an insertion indicator.
'a'+'p'='ap'
'ap' inserted inside 'cable' is 'CAPABLE'.
'being' is the link.
(Other definitions for capable that I've seen before include "'Clever, able (7)'" , "Efficient" , "Able, proficient" , "Adept" , "qualified" .)