Key is with lock in Harrow (8)
I believe the answer is:
distress
'harrow' is the definition.
(thesaurus)
'key is with lock' is the wordplay.
'key' becomes 'd' (musical key).
'with' says to put letters next to each other.
'lock' becomes 'tress' (both can mean a piece of hair).
'd'+'is'+'tress'='DISTRESS'
'in' is the link.
(Other definitions for distress that I've seen before include "Deep anxiety" , "Adversity" , "Unhappiness" , "Pain, trouble or sorrow" , "Mental or emotional pain" .)