The Mousetrap benefitted from this extended ladder (4,3)
I believe the answer is:
long-run
'the mousetrap benefitted from this' is the definition.
(The Mousetrap had a record-breaking West End run)
'extended ladder' is the wordplay.
'extended' becomes 'long-' (similar in meaning).
'ladder' becomes 'run' (synonyms).
'long'+'run'='LONG-RUN'
(Other definitions for long-run that I've seen before include "In the -- means at the final result" , "Period of time sufficient for things to work themselves out" , "In the - - is finally" .)