Certain to use lab for a change (8)
I believe the answer is:
absolute
'certain' is the definition.
(I've seen this before)
'to use lab for a change' is the wordplay.
'for a change' is an anagram indicator.
'to'+'use'+'lab'='touselab'
'touselab' with letters rearranged gives 'ABSOLUTE'.
(Other definitions for absolute that I've seen before include "decided" , "Pure or free from restrictions" , "Complete or unconditional" , "for certain" , "Unrestricted, out-and-out" .)