Back on right lines embracing sweetheart (7)
I believe the answer is:
reverse
'back' is the definition.
(thesaurus)
'right lines embracing sweetheart' is the wordplay.
'right' becomes 'r' (common abbreviation).
'lines' becomes 'verse' (I've seen this before**).
'embracing' indicates putting letters inside.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'r'+'verse'='rverse'
'rverse' placed around 'e' is 'REVERSE'.
'on' is the link.
(Other definitions for reverse that I've seen before include "Turn back to front" , "Invert" , "Go backwards" , "Severer (anag.)" , "Back (of a coin or medal)" .)