Secret I'd discovered initially guarded by female (6)
I believe the answer is:
hidden
'secret' is the definition.
(thesaurus)
'i'd discovered initially guarded by female' is the wordplay.
'initially' suggests taking the first letters.
'guarded by' means one lot of letters goes inside another.
'female' becomes 'hen' (hen is a kind of female**).
The first letter of 'discovered' is 'd'.
'id'+'d'='idd'
'idd' going inside 'hen' is 'HIDDEN'.
(Other definitions for hidden that I've seen before include "Not accessible to view" , "Covert, obscured" , "Concealed, placed out of sight" , "Deliberately placed out of sight and hard to find" , "Latent" .)