Actor heading for theatre, in shape after running (8)
I believe the answer is:
thespian
'actor' is the definition.
('thespian' can be a synonym of 'actor')
'heading for theatre in shape after running' is the wordplay.
'heading for' says to take the initial letters.
'after' means one lot of letters go next to another.
'running' indicates an anagram.
The initial letter of 'theatre' is 't'.
'in'+'shape'='inshape'
'inshape' anagrammed gives 'hespian'.
't'+'hespian'='THESPIAN'
(Other definitions for thespian that I've seen before include "stage performer" , "Stage actor" , "Theatrical performer" , "Dramatic actor" , "He paints (anag)" .)