In essay, side penned (6)
I believe the answer is:
trendy
'in' is the definition.
('in' can mean trendy or fashionable)
'essay side penned' is the wordplay.
'essay' becomes 'try' (synonyms).
'side' becomes 'end' (I can't explain this - if you can you should believe this answer much more).
'penned' indicates putting letters inside (inserted letters are enclosed or penned in).
'try' enclosing 'end' is 'TRENDY'.
Can you help me to learn more?
(Other definitions for trendy that I've seen before include "following latest convention" , "Fashionable, in style" , "Consciously fashionable" , "'Hip, fashionable (6)'" , "In fashion, of popular style" .)