Cross lecturer had dug out some insulation (7,8)
I believe the answer is:
draught excluder
'some insulation' is the definition.
'cross lecturer had dug out' is the wordplay.
'cross' becomes 'x'.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'x'+'lecturer'+'had'+'dug'='xlecturerhaddug'
'xlecturerhaddug' is an anagram of 'DRAUGHT EXCLUDER'.
(Other definitions for draught excluder that I've seen before include "Current bar" , "that might stop delivery of beer?" , "Solution to current problem" .)