Bishop to equip jail (4)
I believe the answer is:
brig
'jail' is the definition.
(I've seen this in another clue)
'bishop to equip' is the wordplay.
'bishop' becomes 'B' (chess abbreviation).
'to equip' becomes 'rig' (rigging is a kind of equiping).
'b'+'rig'='BRIG'
(Other definitions for brig that I've seen before include "Sailing-vessel" , "A ship; army officer (abbr.)" , "officer contracted" , "staff officer briefly used" , "Two-masted square-rigged vessel" .)