Trendy regular sweetheart, not unknown as an alternative (7)
I believe the answer is:
instead
'as an alternative' is the definition.
(synonyms)
'trendy regular sweetheart' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'regular sweetheart' becomes 'stead' (I can't justify this - if you can you should believe this answer much more).
'in'+'stead'='INSTEAD'
'not unknown' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for instead that I've seen before include "In lieu" , "In place (of)" , "for a change" , "use this as a replacement" , "Rather - detains (anag)" .)