One marsupial in love with another (7)
I believe the answer is:
opossum
'one marsupial' is the definition.
(I know that opossum is a type of marsupial)
'in love with another' is the wordplay.
'in love' becomes 'possum' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'with' means one lot of letters go next to another.
'another' becomes 'o' (I've seen this in other clues).
'possum' after 'o' is 'OPOSSUM'.
Can you help me to learn more?
(Other definitions for opossum that I've seen before include "Native of America" , "American and Australian marsupial, often with the first letter dropped" , "Tree climber" , "American tree-dweller" , "New World marsupial" .)