Cross lecturer had dug out some insulation (7,8)
I believe the answer is:
Here is my best explanation:
'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).
'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?" and "Solution to current problem".)