A long run largely devised to produce weariness (7)
I believe the answer is:
languor
'weariness' is the definition.
(I've seen this before)
'a long run largely devised' is the wordplay.
'largely' means to remove the last letter.
'devised' is an anagram indicator.
'a'+'long'+'run'='alongrun'
'alongrun' with its last letter taken off is 'alongru'.
'alongru' with letters rearranged gives 'LANGUOR'.
'to produce' is the link.
(Other definitions for languor that I've seen before include "Weariness; relaxation" , "Pleasurable inactivity" , "No enthusiasm" , "Enervation" , "Mental laziness" .)