Taken separately — it could be passionate (6)
I believe the answer is:
ardent
'be passionate' is the definition.
(similar in meaning)
'taken separately it' is the wordplay.
I cannot quite see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
The remaining letters 'arden' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
'could' is the link.
Can you help me to learn more?
(Other definitions for ardent that I've seen before include "Fiery and zealous" , "Ranted, being passionate" , "full of zeal" , "Passionate, fervent" , "Enthusiastic or passionate" .)