In theatre, a cleaner grabs sticky substance (7)
I believe the answer is:
treacle
'sticky substance' is the definition.
(sticky syrup)
'in theatre a cleaner grabs' is the wordplay.
'in' says the answer is hidden in the clue.
'TREACLE' can be found hidden inside 'theatre a cleaner grabs'.
(Other definitions for treacle that I've seen before include "Carroll's sisters had to live with" , "Contrived sentimentality - sticky substance" , "flattery" , "well described by Dormouse" , "Sticky produce" .)