Sweetheart has reason to be passionate (7)
I believe the answer is:
emotive
'passionate' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I don't see?
'sweetheart has reason' is the wordplay.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'has' means one lot of letters go next to another.
'reason' becomes 'motive' (I've seen this before).
'e'+'motive'='EMOTIVE'
'to be' acts as a link.
Can you help me to learn more?
(Other definitions for emotive that I've seen before include "Move tie (anag)" , "Arousing much feeling" , "Generating feelings" , "Arousing deep feeling" , "Tending to arouse feelings" .)