Sore back precludes fencing (5)
I believe the answer is:
ulcer
'sore' is the definition.
(I've seen this before)
'back precludes fencing' is the wordplay.
'back' says the letters should be written backwards.
'fencing' indicates a hidden word (some letters fenced in).
'reclu' can be found hidden inside 'precludes'.
'reclu' written backwards gives 'ULCER'.
(Other definitions for ulcer that I've seen before include "Cruel (anag.)" , "Cruel sort of body sore" , "Cruel kind of body sore" , "Inflammatory internal or external lesion" , "Spot of bother" .)