Tiresomely long finish, not so great (7)
I believe the answer is:
endless
'tiresomely long' is the definition.
(I've seen this before)
'finish not so great' is the wordplay.
'finish' becomes 'end' ('end' can be a synonym of 'finish').
'not so great' becomes 'less'.
'end'+'less'='ENDLESS'
(Other definitions for endless that I've seen before include "Infinite, perpetual" , "With no limit in sight" , "Eternal - unbroken" , "Non-stop" , "'Eternal, perpetual (7)'" .)