You are soundly invested in a partner's lovers (8)
I believe the answer is:
amateurs
'a partner's lovers' is the definition.
The definition and answer can be both people as well as being plural nouns.
Maybe there's an association between them I don't understand?
'you are soundly invested in' is the wordplay.
'you are' becomes 'ur' (text or internet abbreviation).
'soundly invested' becomes 'amates' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' is an insertion indicator.
'ur' going into 'amates' is 'AMATEURS'.
Can you help me to learn more?
(Other definitions for amateurs that I've seen before include "Dilettantes" , "Tamer USA (anag.)" , "Non-professionals" , "Non-pros" , "They're enthusiastic" .)