Thing is, sweetheart, boss is busy working (9)
I believe the answer is:
obsession
'thing' is the definition.
(I've seen this before)
'sweetheart boss is busy working' is the wordplay.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'busy' is an anagram indicator.
'working' becomes 'on'.
'e'+'boss'+'is'='ebossis'
'ebossis' is an anagram of 'obsessi'.
'obsessi'+'on'='OBSESSION'
'is' is the link.
(Other definitions for obsession that I've seen before include "Extreme interest" , "Compulsion, fixation" , "Mania" , "Fan may display this" , "Complex" .)