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 can't understand how they can define each other.
'when he turns up' is the wordplay.
I cannot really 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 leftover letters.
No letters remain.
This explanation may well be incorrect...
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" .)