Is in the lab, when he turns up (5)
I believe the answer is:
basil
'is in the lab when he turns up' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'when he turns up' is the wordplay.
I cannot quite see how this works, but
'when' could be 'as' (eg 'when I walked' means 'as I walked') and 'as' is present in the answer.
'he' could be 'bil' and 'bil' is present in the remaining letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for basil that I've seen before include "Culinary plant" , "Seasoning" , "Herb - boy's name" , "Saint" , "Man" .)