Large plant in theatre entrance (4)
I believe the answer is:
tree
'large plant' is the definition.
(I've seen this before)
'in theatre entrance' is the wordplay.
'in' indicates a hidden word.
'TREE' is hidden in the letters of 'theatre entrance'.
(Other definitions for tree that I've seen before include "Ash or elm, for example" , "A number" , "Big plant" , "Elder, possibly" , "Conifer, for example" .)