Not a heavy responsibility in the theatre (8)
I believe the answer is:
lighting
'theatre' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'not a heavy responsibility in the' is the wordplay.
I cannot really see how this works, but
'in' is found in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for lighting that I've seen before include "See 14" , "Visual aids" , "What bulbs can provide" , "Starting a fire" , "landing" .)