Gall bladder's sides fool... (5)
I believe the answer is:
brass
'gall' is the definition.
(I've seen this before)
'bladder's sides fool' is the wordplay.
'sides' suggests removing the centre (only the letters on the sides are used).
'fool' becomes 'ass' (both can mean a stupid person).
'bladder' with its centre removed is 'br'.
'br'+'ass'='BRASS'
(Other definitions for brass that I've seen before include "Metal alloy" , "Common alloy" , "Descriptive of 7's instrument" , "Nerve (but not of steel!)" , "It's there, if muck's there" .)