Old lover allowed to keep quiet, that's clear (8)
I believe the answer is:
explicit
'clear' is the definition.
(thesaurus)
'old lover allowed to keep quiet' is the wordplay.
'old lover' becomes 'ex' (I've seen this in other clues).
'allowed' becomes 'licit' (I've seen this before).
'to keep' means one lot of letters goes inside another.
'quiet' becomes 'p' (music symbol for 'piano').
'ex'+'licit'='exlicit'
'exlicit' enclosing 'p' is 'EXPLICIT'.
'that's' acts as a link.
(Other definitions for explicit that I've seen before include "Clip exit, being quite detailed and definite" , "Clearly observable" , "Clear and without concealment" , "Clearly depicted" , "Stated or shown directly, not ambiguous" .)