Quiet remark possibly said by sweetheart (5)
I believe the answer is:
aside
'quiet remark' is the definition.
'aside' can be an answer for 'remark' (I have seen 'Confidential remark' mean 'aside' so perhaps 'remark' could also mean 'aside'). I'm not sure about the 'quiet' bit.
'possibly said by sweetheart' is the wordplay.
'possibly' is an anagram indicator.
'by' means one lot of letters go next to another.
'sweetheart' becomes 'e' (the middle letter or heart of 'sweet').
'said' with letters rearranged gives 'asid'.
'asid'+'e'='ASIDE'
Can you help me to learn more?
(Other definitions for aside that I've seen before include "Where one's taken for intimacy" , "remark, incidentally" , "In reserve -- confidential remark" , "Privately, apart" , "Words spoken apart in an undertone" .)