Member some fool I'm backing (4)
I believe the answer is:
limb
'member' is the definition.
(member can mean a limb)
'some fool i'm backing' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'LIMB' is hidden in the letters of 'fool im backing'.
(Other definitions for limb that I've seen before include "Part of body or branch of tree" , "Eg, arm, leg" , "Substantial branch from a tree trunk" , "Jointed appendage" , "Large tree branch" .)