Part of the target is a small truck in the theatre (5)
I believe the answer is:
outer
'part of the target' is the definition.
(outer ring of a darts board)
'small truck in the theatre' is the wordplay.
'small truck' becomes 'ute' (Australian abbreviation for utility vehicle; 'small' suggests a short form).
'in the' indicates putting letters inside.
'theatre' becomes 'or' (OR for Operating Room - surgical theatre).
'ute' put into 'or' is 'OUTER'.
'is a' acts as a link.
(Other definitions for outer that I've seen before include "Poor shot" , "Ring furthest from the bullseye" , "Exterior or external, like space" , "Objective" , "External; peripheral" .)