Lads running wild outside one cheap bar (8)
I believe the answer is:
disallow
'bar' is the definition.
(I've seen this before)
'lads running wild outside one cheap' is the wordplay.
'running wild' indicates an anagram.
'outside' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'cheap' becomes 'low'.
'lads' anagrammed gives 'dsal'.
'dsal' going around 'i' is 'disal'.
'disal'+'low'='DISALLOW'
(Other definitions for disallow that I've seen before include "Refuse to accept as valid, as judge might a claim" , "refuse permission" , "Rule out" , "Prevent" , "Prohibit, veto" .)