Unapproachable science lab is redesigned (12)
I believe the answer is:
inaccessible
'unapproachable' is the definition.
(similar in meaning)
'science lab is redesigned' is the wordplay.
'redesigned' indicates an anagram.
'science'+'lab'+'is'='sciencelabis'
'sciencelabis' with letters rearranged gives 'INACCESSIBLE'.
(Other definitions for inaccessible that I've seen before include "hard to reach" , "Unreachable" , "Difficult to reach" , "Impossible to reach, understand" , "Unable to be reached" .)