Dismantle a United Nations oil installation? (5)
I believe the answer is:
unrig
'dismantle' is the definition.
(I've seen this before)
'a united nations oil installation?' is the wordplay.
'a united nations' becomes 'UN' (abbreviation).
'oil installation?' becomes 'rig' (as in an oil rig).
'un'+'rig'='UNRIG'
(Other definitions for unrig that I've seen before include "Take down sails" , "Take tackle out" , "In rug (anag.)" , "Dismantle" , "(Of a boat) strip of its mast, stays etc" .)