Fanatical bishop hiding in sack (5)
I believe the answer is:
rabid
'fanatical' is the definition.
(I know that fanatic can be written as rabid)
'bishop hiding in sack' is the wordplay.
'bishop' becomes 'B' (chess abbreviation).
'hiding in' means one lot of letters goes inside another.
'sack' becomes 'raid' (I've seen this in other clues).
'b' going within 'raid' is 'RABID'.
(Other definitions for rabid that I've seen before include "Burning" , "Frenzied" , "Mad like deranged dog" , "Affected by hydrophobia" , "Extremely fanatical" .)