Retired father, one with bug (7)
I believe the answer is:
wiretap
'bug' is the definition.
(I know that bug can be written as wiretap)
'retired father one with' is the wordplay.
'retired' says the letters should be written in reverse.
'father' becomes 'pater' (old-fashioned term for father).
'one' becomes 'i' (Roman numeral).
'with' becomes 'w' (short for with).
'pater'+'i'+'w'='pateriw'
'pateriw' in reverse letter order is 'WIRETAP'.
(Other definitions for wiretap that I've seen before include "bug" , "sneaky listen" , "Eavesdrop" , "cloak-and-dagger arrangement?" , "eavesdropping" .)