Finish top of theatre first - that's a good thing (7)
I believe the answer is:
godsend
'good thing' is the definition.
(thesaurus)
'finish top of theatre first' is the wordplay.
'finish' becomes 'end' (synonyms).
'top of theatre' becomes 'gods' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'first' means one lot of letters go next to another.
'end' put after 'gods' is 'GODSEND'.
'that's a' acts as a link.
Can you help me to learn more?
(Other definitions for godsend that I've seen before include "Welcome good fortune" , "Something very opportune" , "Lucky find" , "Unexpected help" , "welcome acquisition" .)