Mind check and vacate the set after Matilda stands in line, they say (2,4)
I believe the answer is:
iq test
'mind check' is the definition.
(test of one's intelligence)
'vacate the set after matilda stands in line they say' is the wordplay.
I cannot really understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
an anagram of 'set' is 'est' which is located in the answer.
'matilda' could be 'i' (where the crossword setter is 'Matilda') and 'i' is found in the answer.
A single letter 'q' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'and' is the link.
(Other definitions for iq test that I've seen before include "Psychometric measurement of intelligence" , "Intelligence trial" , "'A measurement of intelligence (2,4)'" , "Measure of intelligence quotient" .)