About poem being in secret writing (4)
I believe the answer is:
code
'secret writing' is the definition.
(I've seen this in another clue)
'about poem' is the wordplay.
'about' becomes 'c' (circa).
'poem' becomes 'ode' (ode is a type of poem).
'c'+'ode'='CODE'
'being in' is the link.
(Other definitions for code that I've seen before include "System of signals" , "Cryptographic system" , "Cipher - set of principles" , "Program instruction" , "Collection of laws" .)