a sorcerer, was (when hidden) longing (5)
I believe the answer is:
witch
'a sorcerer' is the definition.
(I've seen this before)
'was when hidden longing' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'hidden' suggests deleting specific letters.
'longing' becomes 'itch' (itch can mean a desire or longing).
'was' with 'as' taken away is 'w'.
'w'+'itch'='WITCH'
(Other definitions for witch that I've seen before include "Female practising black magic" , "Female sorcerer" , "Bad speller!" , "... and magician" , "Enchantress" .)