Sadly ended day mostly in blind alley (4,3)
I believe the answer is:
dead end
'blind alley' is the definition.
(synonyms)
'sadly ended day mostly' is the wordplay.
'sadly' indicates anagramming the letters.
'mostly' means to remove the last letter.
'ended'+'day'='endedday'
'endedday' with its last letter taken away is 'endedda'.
'endedda' is an anagram of 'DEAD END'.
'in' is the link.
(Other definitions for dead end that I've seen before include "Blind alley, cul-de-sac" , "A blind alley, sounds moribund" , "Where no progress can be made" , "that's leading nowhere" , "A blind alley, point of no further progress" .)