Solve cryptic by working with Guardian's first serenade (4,4)
I believe the answer is:
love song
'serenade' is the definition.
(a serenade might be sung to a lover)
'solve cryptic by working with guardian's first' is the wordplay.
'cryptic' indicates an anagram (a cryptic or obscure order of the letters).
'by' means one lot of letters go next to another.
'working' becomes 'on' (on can mean working or activated).
'with' is a charade indicator (letters next to each other).
'guardian's first' becomes 'g' (1st letter of 'guardian').
'solve' is an anagram of 'loves'.
'loves'+'on'+'g'='LOVE-SONG'
(Other definitions for love song that I've seen before include "Romantic tune" , "Serenade or similar lyric" , "air of romance?" .)