Lover’s song — no advert for calm (6)
I believe the answer is:
serene
'for calm' is the definition.
(synonyms)
'lover's song no advert' is the wordplay.
'lover's song' becomes 'serenade' (I've seen this in another clue).
'no' suggests deleting specific letters.
'advert' becomes 'ad' (synonyms).
'serenade' with 'ad' taken away is 'SERENE'.
(Other definitions for serene that I've seen before include "Tranquil, unruffled" , "still" , "Calm and composed" , "Calm and unruffled, like a princely title." , "Peaceful and unruffled" .)