Keep out of steep enclosure for entry to barbican (5)
I believe the answer is:
debar
'keep out' is the definition.
(to debar someone is to keep them out of something)
'steep enclosure for entry to barbican' is the wordplay.
'steep' becomes 'dear' (both can mean expensive).
'enclosure for' is an insertion indicator (inserted letters are enclosed).
'entry' suggests taking the first letters (I've seen 'entry of' mean this).
The initial letter of 'barbican' is 'b'.
'dear' going around 'b' is 'DEBAR'.
'of' acts as a link.
(Other definitions for debar that I've seen before include "Stop" , "Keep out" , "Formally prevent" , "Block" , "Exclude, prohibit" .)