Deal with, in theatre, a thespian (5)
I believe the answer is:
treat
'deal with' is the definition.
(synonyms)
'in theatre a thespian' is the wordplay.
'in' says the answer is hidden in the clue.
'TREAT' is hidden within 'theatre a thespian'.
(Other definitions for treat that I've seen before include "Give medical care or free meal to" , "Care for medically" , "Doctor, manage" , "Act toward or regard in a certain way" , "Behave towards" .)