Relations with pleasant son involving sweetheart (6)
I believe the answer is:
nieces
'relations' is the definition.
(niece is a kind of relation)
'pleasant son involving sweetheart' is the wordplay.
'pleasant' becomes 'nice' (similar in meaning).
'son' becomes 's'.
'involving' indicates putting letters inside.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'nice'+'s'='nices'
'nices' enclosing 'e' is 'NIECES'.
'with' is the link.
(Other definitions for nieces that I've seen before include "Scene I set up for relatives" , "They're family" , "They're related" , "Relations" , "Young female relatives" .)